


J *S 

Class__._iL_ 

Book. LJ~ 5 5 



COPYRIGHT DEPOSIT. 


GPO 























A LABORATORY MANUAL 
OF PHYSICS 


- Aa 

HOMER L. DODGE, Ph.D. 

Dean of the Graduate School and Professor of Physics at the 
University of Oklahoma 


AND 

DUANE ROLLER, M. S. 

Instructor of Physics at the University of Oklahoma 




Harlow Publishing Company 

Oklahoma City, Oklahoma 





(?C 36- 

."£> 6 \ 3 * 


i (V, ■.. 



Copyright 1926 by 
Harlow Publishing Co. 








© Cl A 901947 

SEPI.V26 

0 

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PREFACE 


This book has been written to satisfy the need for a suitable laboratory 
manual of general physics for use in the schools of Oklahoma. It contains 
the thirty-seven experiments which have been approved as a “minimum 
list” by the State Department of Education and which are outlined in the 
“'Course of Study In Science.” It also contains the ten additional experi¬ 
ments which are recommended for schools having the necessary apparatus. 
A few changes have been made in the apparatus and in the directions for 
performing the experiments, where the resulting advantages were too 
great to be disregarded. 

This manual differs from similar books in several respects: (1) It 
provides a comprehensive course in elementary laboratory physics without 
requiring expensive apparatus. (2) Questions are used extensively and, 
instead of being grouped at the end of the experiment, are placed at the 
points in the experiment where they naturally arise. Each question should 
be answered by the student at the time that it is encountered in perform¬ 
ing the experiment, so that the student will keep clearly in mind just what 
he is attempting to do and just how he should interpret his results. (3) 
Preceding the directions for each experiment is a brief introduction which 
provides an incentive for performing the experiment and also the funda¬ 
mental facts which the student should have in order that he may realize 
the maximum benefit from it. (4) The manual contains a complete list 
of the apparatus needed for each experiment. Each list is designed to 
assist the instructor in preparing for the laboratory period in the minimum 
amount of time. 

Special experiments requiring little or no additional apparatus have 
been introduced at various nlaces throughout the book. These experiments 
should prove attractive to the student who has the ability and the desire to 
to do more work than is outlined by the minimum, list. Because of their 
briefness, these sDecial experiments offer opportunity for the development 
of originality and forethought^in experimentation. 

Many of the experiments have been divided into two or more parts in 
order that they may be adjusted to the length of the laboratory period and 
in order that each problem may be concrete and specific. Consequently 
in several of the experiments certain parts may be omitted if it is found 
desirable. Such omissions should depend upon the interest and training 
of the students and upon the limitations of apparatus. 

It is suggested that each student be given the opportunity to perform 
several of the experiments alone. There is also considerable value in 
having several of the experiments performed by the class as a group, in 
which case the instructor or a student should manipulate the apparatus. 

The appendix includes some material which would ordinarily be found 
in the body of the book. This arrangement facilitates ready reference 
to such topics as Graphs, Balances and Ammeters, and encourages the 
student to develop the habit of referring to sources of information other 
than those collected for the purpose at hand. 



Elementary physics should be a fascinating subject for boys and girls 
and, while it is true that the subject will present difficulties even for the 
best students, there is no reason why a science which has contributed 
so abundantly to every phase of human progress should not be attractive. 
The authors have prepared this laboratory manual with the hope that 
it will assist the teacher in initiating his students into the wonderland of 
physical science and in arousing in them the desire to learn more about 
the “why” of things. 

We take pleasure in thanking our colleagues Dr. William Schriever 
and Mr. Harvey C. Roys who read the manuscript and the proof, and 
we wish to express our appreciation to Mr. John Gilbert who exercised 
much patient care and skill in preparing the drawings. We are also 
indebted to the Welch Scientific Company and to the Central Scientific 
Company for a number of illustrations. 

The authors will be glad to receive criticisms and suggestions from 
the teachers who use this manual. 

Homer L. Dodge, 
Duane Roller. 

Norman, Oklahoma. 


TABLE OF CONTENTS 


Exp. No. , Page 

1. Density of Solids by Weighing and Measuring ____ 1 

2 . Archimedes Principle and the Density of an Irregular Solid. 5 

3. Density of Liquids and of Solids Lighter than Water ...... 8 

4. Boyle’s Law__ ______ 11 

5. The Molecular Structure of Matter ..... 14 

6. Parallel Forces and the Principle of Moments_ ____ 15 

7. Concurrent Forces __ _____ _ 18 

8. The Simple Pendulum_ _______ 21 

9. Uniformly Accelerated Motion ....... 24 

10. Blocks and Tackle .... .... .. 26 

11. Friction _____________ 29 

12. Testing a Thermometer__ _______ 32 

13. Dew Point and Relative Humidity..-.-_ ____ 34 

14. Coefficient of Linear Expansion__ 36 

15. Specific Heat . 38 

16. The Mechanical Equivalent of Heat ___ 41 

17. Change of State .. 43 

18. Heat of Fusion of Ice__ 44 

19. Speed of Sound . 46 

20. Wave Length of the Note of a Tuning Fork _ 48 

21. Magnetic Fields __ 50 

22. The Nature of Magnetism ___ 52 

23. Static Electricity . 54 

24. The Voltaic Cell . 56 

25. Magnetic Effects of an Electric Current.. .. 58 

26. Magnetic Properties of Coils ... 60 

27. The Electric Bell __ _ _ __-----.... 62 

28. Electromotive Force and Internal Resistance of Cells... 63 

29. Electrolysis and Electroplating __ 66 

30. Electric Currents by Electromagnetic Induction _ 68 

31. Motors and Generators ... 70 

32. Reflection from a Plane Mirror . 73 

33. Refraction of Light by a Prism . . 76 

34. The Rumford Photometer _ 78 

35. Converging Lenses .......—.. 80 

36. The Magnifying Power of a Simple Microscope-- 83 

37. The Astronomical Telescope -- 85 







































Add. 1. Pressure in a Liquid ---.-. 

Add. -2. Lung-Pressure; Pressure in the City Gas Mams- 

Add. 3. The Inclined Plane . ~ .- 

Add. 4. Effect of Pressure on the Boiling Point. 

Add. 5. The Laws of Vibrating Strings . 

Add. 6. Laws of Resistance Using Ammeter and Voltmeter 

Add. 7. The Wheatstone Bridge ---- 

Add. 8. Efficiency of Incandescent Lamps--- 

Add. 9. Electricity in the Home-- --- 

Add. 10. The Bunsen Photometer- 

SPECIAL EXPERIMENTS 

Density of a Cylindrical Sol'd - 

Density of Milk -- 

Center of Gravity - 1 - 

The Household Thermometer -- 

The Vacuum Bottle - 

Absorbing and. Radiating Surfaces—^. - - - 

Interference of Sound Waves - - - 

Another Method for Calculating Wave Length - 

To Test a Lead Storage Cell - 

Concave Mirrors - 

The Compound Microscope - 

How to Make a Barometer -- 

Measuring Altitude with an Aneroid Barometer - 

School Electric Lighting System -—- 

Candle Power of Lamps - 


Page 

. 87 
. 90 
. 93 
. 95 
. 97 
TOO 
.103 
.107 
.110 
.113 


_ 4 

_ 10 

_ 20 

_ 40 

_ 40 

_43 

_ 47 

_ 49 

_ 72 

_ 75 

_ 84 

_ 92 

_ 90 

_ 1(M) 

_ 114 


Appendix - 

List of Apparatus 


115 

123 






























LABORATORY MANUAL OF PHYSICS 

1. DENSITY OF SOLIDS BY WEIGHING AND MEASURING 


Books on physics, engineering, and chemistry contain tables which 
give the densities of many substances, and these tables are often consulted 
by scientists, engineers and manufacturers. If an engineer knows the 
density of the wood and steel which he uses in building a bridge, he can 
calculate the mass of the structure with an accuracy sufficient for his needs. 

The density of a substance is the mass of a unit volume. This is us¬ 
ually given in grams per cubic centimeter, although in applied sciences 
like agriculture and home economics, it is often more convenient to give 
the density of a substance in pounds per cubic foot, or in pounds per cubic 
inch. 


1. If the mass of a block of wood is 280 g and its volume is 500 

cm 3 , what is the average density of the wood? Ans. 0.56 g/'cm 3 . 

2. Why do we say “average density” in the case of wood, and 

simply “density” in the case of pure lead or iron? 

To measure the density of a substance, we must find both the mass 
and the volume of a sample of the substance, and then divide the former by 
the • latter. The mass can be 
measured on a beam or platform 
balance, but the way in which 
the volume is measured depends 
on the shape of the sample. In 
this experiment, we will deal 
only with samples which are in 
the form of rectangular blocks 
or spheres. 

Exp. 1, Part I. Measure 
the densities of several kinds of 
ivood or metal of which you have 
samples in the form of rectangu¬ 
lar blocks. 

To find the volume of a rec¬ 
tangular block, we must first measure its length, width and thickness with 
a meter stick, and then compute the volume by means of the formula, 

Volume = length X width X thickness. 

Since there is no such thing as a perfectly rectangular block, it is 
necessary to measure the length, width and thickness at different points. 
We can then compute the average length, width and thickness, and from 
these determine the volume of the block. 












2 


Laboratory Manual of Physics 


First obtain the average length of the block. To do this, measure the 
four edges of the block which are parallel to its length, and then find the 
sum of these four measurements and divide this sum by four. Each 
measurement should be made to a hundredth of a centimeter and should be 
written down in centimeters and a decimal fraction thereof. Since a 
hundredth of a centimeter is one tenth of the smallest division on an ordi¬ 
nary meter stick, it will be necessary to estimate to tenths of the smallest 
division. The ability to estimate distances on a scale to tenths of the 
smallest division is very important in making measurements. 

There will be less error in these measurements if the meter stick is 
placed on the block so that the scale divisions touch the wood, as in Fig. 1. 
The ends of the meter stick must not be used, unless these ends are tipped 
with metal. Why? 

The average width and the average thickness of the block are obtained 
in the same way as the average length, in each case making four measure¬ 
ments. 

Make a table for your data similar to the one at the end of this ex¬ 
periment, and record the result of each measurement just as soon as it is 
obtained. Compute the volume of the block and then proceed to measure 
its mass on a beam or platform balance. A description of various types 
of balances and directions for their use are given under Balances in the 
appendix. 

Exp. 1, Part II. Measure the density of steel, glass or some other 
substance of which you have a sample in the form of a sphere. 

To find the volume of a sphere, measure its diameter d and then com¬ 
pute its volume V, using the formula, 

v = y d 3 . 

The diameter is best measured with a micrometer caliper. A descrip¬ 
tion of the micrometer caliper and directions for its use are given under 
Calipers in the appendix. 

First determine the zero reading of the micrometer caliper by the 
method explained in the appendix. This reading, if different from zero, 
must be added to, or subtracted from, all future readings made with the 
instrument. Then make, say, four measurements with the calipers of the 
diameter d of the sphere. Each measurement should be made to a thou¬ 
sandth of a centimeter and should be written down in centimeters and a 
decimal fraction thereof. A thousandth of a centimeter is the smallest 
reading which can be made on an ordinary micrometer caliper. 

Record each measurement as soon as it is made in a table similar to 
the one at the end of this experiment. Never record data on scratch 
paper. 

Obtain the average diameter of the sphere by finding the sum of the 
four measurements and dividing by four. Compute the volume of the 
sphere and then proceed to measure its mass on a beam or platform bal¬ 
ance. A description of various types of balances and directions for their 
use are given under Balances in the appendix. 


Laboratory Manual of Physics 


3 


Note. If a micrometer caliper is not available, measure the diameter of the sphere 
with a meter stick. In this case it will be found best to use two identical spheres, placing 
them side by side between two blocks, as in Fig. 2. Measure the distance between the 
blocks at each end and at a level with the centers of the spheres. Then shift the blocks 
and repeat this procedure, obtaining four measurements in all. 


Each measurement should be made to a hundredth of a centimeter and should be 
written down in centimeters and 
a decimal fraction thereof. Since 
a hundredth of a centimeter is 
one-tenth of the smallest division 
of an ordinary meter stick, it 
will be necessary to estimate to 
tenths of the smallest division. 

There will be less error in 
these measurements if the meter 
stick is placed against the blocks 
so that the scale divisions touch 
the wood, Fig. 1. The ends of the 
meter stick must not be used, 
unless they are tipped with metal. 



Obtain the average diameter of one of the spheres by finding the sum of the four 
measurements and dividing by four. Compute the average volume of one sphere and then 
proceed to measure its mass on a beam or platform balance, directions for the use of 
which are given under Balances in the appendix. 


Suggested form of record 


Part I 


Substance 


Trial 

Length 
in cm 

Width 
in cm 

Thickness 
in cm 

1 




2 




Q 

o 




4 




Average 


- 



Sample No. 


SUMMARY OF RESULTS 

Substance and 
sample number 

Density in 

g /cm 3 








Volume 


( 


) X ( 


) X ( 


) 


cm 


3 


Mass 


<r 
^ • 


Density = 


( 


) ^ ( 


) 


*>/ 


cm 


(Make a similar table for each block tested.) 


















































4 


Laboratory Manual of Physics 


Part II 

Substance _ Sample No - 

Diameter of spherical sample: 

Trial 1 _ cm ; 2 _cm; trial 3 _cm; trial 4 -cm. 

Average diameter ---cm. # 

3.1416 X ( _) 3 

Volume of sample = - ; - —-—-cm . 

6 

Mass of the sample-g. 

Density = (-,) m' (-) ss ------ S/cm*. 


Special Experiment. Find the density of a substance of which you 
have a sample in the form of a cylinder. Measure the dimensions of the 
cylinder with a vernier caliper, the directions for the use of which will be 
found under Calipers in the appendix. 

Make three measurements of the length and three of the diameter of 
the cylinder, compute the average length l and the average diameter d, and 
then find the volume, using the formula, 

v - x d% X l 
4 

Use a beam or platform balance to measure the mass of the cylinder 
and then calculate the density of the substance in the usual way. 
















2. ARCHIMEDES’ PRINCIPLE AND THE DENSITY OF AN 
IRREGULAR SOLID 


The density of coal, stone and similar substances is obtained by find¬ 
ing the mass and volume of a sample of the substance, and then dividing 
the mass by the volume. 

It is difficult to obtain samples of such substances which are regular 
in shape and hence a meter stick or calipers cannot be used to find their 
volumes. 

The volume of an irregular object can be found, however, by a method 
which is based on Archimedes’ principle. The object is weighed in air and 
then in some liquid of known density, generally water, and its apparent 
loss of weight is noted. This “loss of weight” is, by Archimedes’ principle, 
equal to the weight of the liquid displaced by the object. If the weight 
of the displaced liquid is divided by its density, the volume of displaced 
liquid is obtained; and this is also the volume of the object. 

1. If a piece of granite weighs 180 g less in water than it does in 

air, what is the weight of the displaced water? Ans. 180 g. 

2. In the above question, what is the volume of displaced water? 

Of the granite? (Density of water equals 1 g/cm 3 .) 

3. What is the density of this granite if its mass is 468 g? 


Exp. 2, Part I. Verify Archimedes ’ principle. 

Archimedes’ principle can be verified by finding the loss of weight 
in water of some object of regular shape and comparing this with the 
weight of the water displaced by the object. 

Make the initial adjustments of the beam balance 1 as directed under 
Balances in the appendix. Weigh in air an aluminum 
cylinder or other regular object of fairly large volume. 
Then suspend it in a vessel of water, Fig. 3, and again 
find its weight. Be sure that the vessel of water does 
not touch the pan supports, and that the object is com¬ 
pletely immersed and free from air bubbles. Why? 

The difference between the above weights is the 
loss of weight in water. It is this value which we wish 
to compare with the weight of the displaced water. 

To find the weight of the displaced water 2 , multiply 

1 A spring balance may be used, if necessary. Before making any 
weighings, hold the balance in a vertical position and note the read¬ 
ing of the balance with zero load. This reading, if different from 
zero, must be added or subtracted, as the case may be, from all future 
readings taken with the balance in question. 

2 An alternative method for measuring the weight of. the dis¬ 
placed water is to fill an overflow can (or steam boiler) with water 
until it flows out the spout. Place an empty weighed beaker under the 
spout and carefully lower the object into the overflow can by means 
of a thread. When the last drop of liquid has overflowed, weigh the 
Fig. 3. Archimedes' beaker and water and compute the weight of the water displaced by 
principle. the object. 



[ 5 ] 













6 


Laboratory Manual of Physics 


its volume by its density. The volume is the same as that of the immersed 
object, and since the latter is regular in shape, it can be found with cali¬ 
pers or a meter stick. If the object is rectangular or spherical in shape, 
get the volume by one of the methods given in Exp. 1. If it is a cylinder, 
make four measurements of its length and four of its diameter with a 
vernier caliper \ and compute the average length l and the average diame¬ 
ter d then find the volume of the cylinder, using the formula, 

7 r x d 2 3 4 5 x l 

4 

If a vernier caliper is not available, measure the diameter with a 
meter stick, using a method similar to the one for measuring a sphere, 
given in Exp. 1, Part II. 

4. Compare the loss of weight in water with the weight of the 
displaced water and state in your own words the principle thus 
verified. 

5. Calculate the percent of difference between your values for 
the loss of weight and for the weight of displaced water, using 
the formula, 

„ , , difference between values 

Per cent of difference = - ^ - , -X liw- 

either value 


Exp. 2, Part II. Find the density of brass, coal, rock, or some other 
substance of irregular form. 

Weigh in air and then in water a sample of the substance whose den¬ 
sity is to be found, Fig. 3. Observe the precautions given in Part I. 
Record your data in a table similar to the one given at the end of this ex¬ 
periment. 

1. Show that the following formula is correct: 

n _ _ mass _ 

ensi y ] oss 0 f we jg^ j n wa ter * 

2. The volume of an object immersed in water is numerically 
equal to the loss of weight. Why must we say “numerically”? 

3. Why cannot the “loss of weight” method be used to find the 
density of rock salt? 

4. What becomes of the “lost” weight, when an object is im¬ 
mersed in a fluid? 

5. Given a balance, weights and a vessel graduated in cubic cen¬ 
timeters, how would you find the density of an irregular solid? 


See Calipers in appendix. 






Laboratory Manual of Physics 


7 


Suggested form of record 

• 

Part I 

Weight of object in air 

-_g. 

Weight of object in water 


Loss of weight in water 



Volume of immersed object (R ird data for volume as in Exp. 1) 


Weight of displaced water = ( 

— ) X 1 = __ - _ g. 

Part II 

Substance 

Sample No. _ 


Weight in air. 


Weight in water 

-©• 

Loss of weight in water 


Volume of sample = vol. of water = 

( ) 

- — _ _ cm 3 . 

1 


Density = (__) -5- (_) = -g/cm J 

















3. DENSITY OF LIQUIDS AND OF SOLIDS LIGHTER THAN 

WATER 

The density of a liquid like gasoline is one of the indications of its 
quality. Gasoline of high density should not be used in automobile engines 
and if used in an aeroplane engine, a serious accident may result. In some 
states filling stations are required to post the density or specific gravity of 
the gasoline which they have for sale. The specific gravity of a substance 
is the number of times that the substance is denser than water. 

The special device used for measuring the density of a liquid is called 
a hydrometer. This important instrument is used commercially for testing 
milk, gasoline, oils and the liquid in automobile storage batteries. With 
a hydrometer it is easy to determine, for example, whether the milk which 
you buy is being “watered”. The hydrometer makes use of the displace¬ 
ment method for measuring density, a method which is based on Archi¬ 
medes’ principle. 

The displacement method is also used to measure the density of a solid 
lighter than water. Since such a solid will float in water, it is necessary 
to devise a means for keeping the solid under the surface while it is being 
tested 


Exp. 3, Part I. Make a constant-weight hydrometer and use it to 
measure the density of gasoline. 



Fig. 4. Con¬ 
stant-weight 
hydrometer. 


Place the gasoline or other liquid to be tested in a deep 
vessel. Float a glass tube, about 50 cm X 2 cm, closed at one 
end, in the liquid and load this tube with shot until it sinks to 
within about 2 cm of the top, Fig. 4. 

Place a rubber band or thread around the tube at the exact 
point where it meets the surface of the liquid. Remove the tube 
from the liquid, wipe it dry and measure the length immersed. 
Call this length l x . 

Now float the tube with its contents in a vessel of water, 
mark the point to which it sinks and again measure the length 
immersed. Call this length l w . 

Record your data in a table similar to the one at the end of 
this experiment. The room temperature should be read and in¬ 
cluded in the record because the density of a liquid varies con¬ 
siderably with temperature. 

The density d x of the liquid tested is l w /l x . The tube sank 
both in the water and in the liquid tested until it displaced its 
own weight. Hence the weight of water displaced equaled the 
weight of the unknown liquid displaced, and if the cross-sec¬ 
tional area of the tube is a, 

k X a, X e4 —..Zw X CL x 1, 



[8] 








Laboratory Manual of Physics 


9 


Compute d x and check your result by testing the liquid with a commer¬ 
cial constant-weight hydrometer, if one is available. 

1. Automobile gasoline should have a density of about 0.75 g/cm * 1 2 3 
or less. Is the sample tested suitable for an automobile engine? 

2. Why does the formula l w /l x give the density in grams per cubic 
centimeter ? 

3. How would you change this formula to get the density in 
pounds per cubic foot? 

4. What is the specific gravity of the sample of gasoline tested? 

5. Given a bottle weighing 30 g when empty and 80 g when filled 
with water, what would it weigh if filled with the gasoline which 
you tested? 

Exp. 3, Part II. Find the density of cherry or some other wood less 
dense than water, or of paraffin, wax or cork. 

The density of a substance can be computed if the mass and volume 
of a sample of the substance are known. 

Obtain a sample of the substance to be tested 
and weigh it in air to get its mass. Next find the 
volume of the sample by observing its loss of weight 
when it is completely immersed in water. -Since the 
sample is lighter than water, it will float unless 
weighted down by a sinker; hence, the following 
steps must be employed: 

(a) Attach the sinker to the sample, hang both 
by a thread from the left arm of the beam balance, 
and weigh with the sinker alone immersed in a jar 
of water, Fig. 5. Be sure that the sinker hangs 
freely in the water and that the jar does not touch 
the pan supports. 

(b) Next weigh when both sample and sinker 
are immersed in the water. 

(c) The difference between these two weighings 
gives the weight of water displaced by the sample 
alone, and hence the volume of the sample in cubic 
centimeters. Why? 

(Since the mass and volume of the sample are now 
His. 5. Density of a solid known, the density can be calculated by the usual 
lighter than water. formula. Record observed and computed data in a 
table similar to the one at the end of this experiment. 

1. Why is the combined weight of the solid and sinker in water 
less than their combined weight when the sinker alone is im¬ 
mersed? Explain how this illustrates the principle of the life 
preserver. 

2. What is the specific gravity of the substance tested? What 

is its density in pounds per cubic foot? 



















10 


Laboratory Manual of Physics 


3. A block of wood weighs 92.4 g. The sinker alone weighs 204 
g in water, while both sinker and block weigh 86.4 g in water. 
Find the density of the wood. 

Suggested form of record 


Part I 

Density of __ at - 

Length immersed in liquid being tested, 7_cm. 

Length immersed in water, Z w _cm. 

Density, d x = (---(-) = — — . —-- 

Value with commercial hydrometer = _g/cm 3 . 


Part II 

Substance tested-^- . 

Mass of sample-„—g. 

Weight, sample in air and sinker in water-g. 

Weight, both sample and sinker in water---.-g. 

Weight of water displaced by sample alone—*---g. 

Volume of sample___._cm*. 

Density of---= (_) -h (. 

--g/cm*. 


°<5. 


•g/cm*. 



Special Experiment. Density of Milk. Find the densities of sam¬ 
ples of milk obtained from several dairies. If a lactometer is not available 
use an ordinary commercial hydrometer. “Unwatered” milk should test 
1.027 to 1.033 g/cm 3 . 

How would more water affect the density of milk? Would the density 
of milk change after it had stood half a day? 



















4. BOYLE’S LAW 


When the pressure on air or any other gas is lowered, the gas expands. 
Thus at high altitudes, where the air pressure is low, the air is so “thin” 
that breathing is difficult, and automobile and aeroplane engines will not 
run properly unless an adjustment for the air is made on the carburetor. 


The relation between the 'pressure and volume of a given mass of gas 
is described in a celebrated principle discovered in 1662 by Robert Boyle. 
Boyle used the apparatus shown in Fig. 6. 

The short closed end of the tube contains a column of air AB which is 
separated from the outside air by the mercury in the bend of the tube. 
When the mercury stands at the same level in both tubes, the pressure on 
the inclosed column of air is evidently that due to the atmosphere alone, 
and it is found by reading the barometer. 

If, however, the mercury level C in the open tube is higher than that 
in the closed tube, the column of inclosed air is under a 
- pressure greater than that of the atmosphere. This pressure 

’ ] is given by the barometer reading plus the difference in levels 

h of the mercury. Thus the pressure on the inclosed air can 
I be increased by pouring in more mercury. If the volume 

AB of the gas is measured for each new pressure, the relation 
1 between the pressure and volume of the gas can then be ob¬ 

tained. 


h 


i 


There is one other factor besides pressure which changes 
the volume of a given mass of gas, and that is temperature. 
When the temperature increases, a gas expands. Hence, in 
studying the relation between pressure and volume alone, 
care must' be taken to see that the gas is not heated or cooled. 

Exp, 4. Find how the volume of a given mass of gas 
changes with pressure, and thus test Boyle's law. 



Fig. 6. Boyle’s 


Arrange the J-tube and meter stick as in Fig. 6, and care¬ 
fully pour mercury into the tube until it stands a few centi¬ 
meters higher in the open arm than in the closed arm. Tip 
the tube so as to allow air to escape from the closed arm until 
the mercury stands at about the same level in both arms. 


law apparatus. 

the temperature 


The apparatus is now ready for the experiment. Record 
and also the reading of the barometer in centimeters and 


hundredths. 

Read to tenths of the smallest scale division the heights of the upper 
end A of the air column and mercury surfaces B and C. The position of a 
mercury surface is read by placing the eye on a level with the mercury and 
reading the top of the surface. Avoid grasping the air column with the 
warm hands and do not take readings immediately after having changed 
its volume, as changes in volume temporarily heat or cool the gas. 


[li] 







12 


Laboratory Manual of Physics 


Compute in centimeters the difference in mercury levels h, and add to 
this the barometer reading in centimeters. This gives the total pressure 
P on the gas. 

Find the length of the air column AB. This length will be a measure 
of the volume V of the inclosed air, if we assume that the bore of the tube 
is constant. 

Now pour in more mercury until the difference in level is increased, 
say, to 10 cm, and again find the pressure and resulting volume of the in¬ 
closed air. Continue this process until the air is compressed to about one- 
half its original volume. 

Reread the barometer and thermometer at the end of the experiment. 
Record observed and computed data in a table similar to the one at the end 
of this experiment. 

1. How do the values for P X V compare? 

2. Did the temperature and atmospheric pressure vary any dur¬ 
ing the experiment? 

If care has been taken to reduce the experimental error as much as 
possible, the product P X V will be found to be approximately constant. 
It is never quite constant, even when there is no error, but for small 
changes in pressure it way be considered constant. With the J-tube it is 
possible to get a series of values for P X V which do not vary by more 
than 2 or 3 percent. 

3. You have found that, approximately, P x Vi = P 2 V 2 —' P 3 V 3 — 

P 1 Vo 

etc. Show, then, that n - = • 

P 2 V 1 

4. Is the pressure directly or inversely proportional to the vol¬ 
ume? 

p Vo 

5. The equation D = is one way of stating Boyle’s Law. 

P 2 V 1 

Express it in words. 

6. Using the average of the series of values for P X V which you 
obtained, compute the relative volume, expressed in centimeters of 
length, of the air inclosed in your J-tube when the pressure is in¬ 
creased to 2.5 atmospheres (2.5 X 76 cm of mercury). 

7. How would your values of P X V have been changed, (a) if a 
greater mass of inclosed gas had been used, (>b) if the room had 
suddenly gotten warmer, (c) if the barometer had dropped dur¬ 
ing the experiment, unknown to you? 


Laboratory Manual of Physics 13 

Suggested form of record 

Barometric pressure at beginning of experiment_cm. 

Barometric pressure at end of experiment _„_cm. 

Temperature at beginning of experiment _°CL 

Temperature at end of experiment _°C. 

Position of upper end of air column A _cm. 


Mercury- 

surface 

B 

Mercury 

surface 

C 

Pressure h 
due to mercury, 
in cm 

Total 

pressure P, 
in cm 

Volume 

V of air 

AB 

P X V 

• 








































| 

































































5. THE MOLECULAR STRUCTURE OF MATTER 


It is now generally supposed that substances are made up of very 
minute particles called molecules. These molecules consist of one or more 
still smaller particles called atoms. Spaces exist between the molecules 
of a substance and this explains, for example, why a gas can be compressed 
into a small space and why two gases can be mixed together. 

There is much evidence that these molecules are in rapid motion, 
though the manner in which they move differs for solids, liquids and gases. 
One evidence that the molecules of a gas are in rapid motion is the rapid¬ 
ity with which a gas will diffuse; the odor from cabbage being boiled in 
the kitchen soon penetrates to all parts of the house. 

Exp. 5, Part I. Show by an experiment that liquids are not continu¬ 
ous in structure, but are made up of discrete particles. 

Half fill the hydrometer tube of Exp. 3 with water. Incline the tube 
at an angle, to prevent mixing, and carefully add alcohol until the tube 
is filled. 

Place the thumb tightly over the open end of the tube, slowly invert 
and note what happens. Continue this rotation of the tube until the liq¬ 
uids are thoroughly mixed, keeping the thumb over the open end. 

1. Describe all that you observed. 

2. Why was your thumb pressed into the tube? 

3. Will a quart of sand and a quart of marbles measure two 
quarts when they are mixed together? 

4. How does the experiment with alcohol and water illustrate the 
molecular constitution of liquids? 

Exp. 5, Part II. Show experimentally that gases diffuse rapidly. 

Procure two tumblers or beakers of about the same width. Wet the 
inside of one with a few drops of ammonia water and that of the second 
with a little hydrochloric acid. Place a cardboard or paper cover over each 
tumbler. 

1. Describe the appearance of the gas in each tumbler. 

Invert the second tumbler over the first, with the paper between 
them, placing them so that the edges will match. Remove the paper. 

2. Describe what you see. 

The two substances have united chemically to form a new substance, 
ammonium chloride, which is not colorless and therefore can be seen. 

3. How does this experiment lend support to the view that the 
molecules passing off from the two evaporating liquids are mov¬ 
ing rapidly in all directions? 

[ 14 ] 


6. PARALLEL FORCES AND THE PRINCIPLE OF MOMENTS 


The ability of a force to move an object depends upon the magnitude 
of the force, and its ability to turn or rotate an object depends upon the 

distance of the force from the axis of 
rotation, as well as upon its magni¬ 
tude. 

The turning ability of a force is 
called its moment. The moment is the 
product of the force and its lever arm, 
the lever arm being the perpendicular 
distance from the axis to the line of ac¬ 
tion of the force. 

Fig. 7. Four parallH^forces acting on a Many cases arise in physics and en¬ 
gineering in which a body in equili¬ 
brium is being acted on by a number of forces, all of which are 'parallel to 
each other. This is true in the case of the bridge shown in Fig. 7. 



Fig. 8 represents a laboratory model of this bridge. The two weights, 
Li and L>, represent the loads on the bridge, and the spring balances the 
supports. The model is in equilibrium; it is prevented from moving and 
from rotating by the parallel forces acting on it. We wish to find how these 
parallel forces are related when the model is kept from moving, and how 
the moments of these forces are related in order to keep it from rotating. 

In order to do this, we shall find by experiment how the sum of the 
upward forces due to the supports compares with the sum of the down¬ 
ward forces due to the loads. 

X 


We shall also find how the 
sum of the moments of force 
tending to produce clock¬ 
wise rotation compares with 
the sum of the moments 
tending to produce counter¬ 
clockwise rotation. 

When we have done this, 
we will know how parallel 
forces and their moments 
always are related in cases 
of equilibrium. 






s 

« 

1 

A 3 C 

f 

O 

L! 

' 1 

\ 1 ' | ' 1 1 1 1 | 

} 1 1 ~ 

u 


Laboratory model of a bridge 


Exp. 6. Set up a model bridge and find the two conditions which exist 
when it is in equilibrium under parallel forces. 

A. Forces. First we shall find how the sum of the forces in one direc¬ 
tion compare with the sum of the forces in the opposite direction, when 
parallel forces are in equilibrium. Arrange the model as shown in Fig. 8. 
For good results the meter stick must be suspended in a horizontal position 
and the two supports, Fi and F 2 , must be accurately parallel. This latter 
condition can be obtained by hanging the spring balances from two nails 


















16 


Laboratory Manual of Physics 


or other supports which are exactly 80 cm. apart and fastening the hooks 
of the balances to the meter stick at the 10 cm and 90 cm marks. Why. 

Record the initial balance readings. These readings are to be sub¬ 
tracted from later readings in order to eliminate the weight of the stick. 

Hang the known loads U and U, from the sliding loops at any two 
points such as B and C. Read each balance and from its reading subtract 
the initial reading. This gives the two upward forces Fi and F 2 . 

Make two more sets of observations with the loads Li and L 2 hung at 
quite different places on the meter stick. 

Compute in each case the sum of the upward forces, Fi and F 2 , and 
the sum of the downward forces, Li and L 2 , and compare their values. 


1. When your model bridge is in equilibrium, how does the sum 
of the forces acting in one direction compare with the sum of the 
forces acting in the opposite direction ? Allow for possible experi¬ 
mental error, which should not exceed 2 per cent. 

2. A 1500 lb. automobile is standing on a 60 ft. bridge which 
weighs 8 tons and which is supported at the ends. What is the 
total force which the supports must exert upward to prevent col¬ 
lapse ? Do the results of your experiment show that this total 
upward force is dependent upon the position of the automobile on 
the bridge ? 

B. Moments. We will next find how the sum of the clockwise mo¬ 
ments compares with the sum of the counter-clockwise moments when par¬ 
allel forces are in equilibrium. If the support D were to break, your model 
bridge would rotate clockwise about A as an axis. We wish to find how 
the moments of the forces are related in order to prevent this rotation. 

Hang the loads Li and L 2 at some convenient distance from the chosen 
axis, A, say 20 cm and 30 cm. Obtain the corrected reading F 2 of the bal¬ 
ance at D and measure its lever arm, AD. Then compute the moment, 
Fo X AD, which tends to produce counter-clockwise rotation about A. Also 
compute the moments, L x X AB, and L 2 X AC, tending to produce clock¬ 
wise motion about the same axis A. 


Make two more sets of observations with the loads, Li and L 2 , hung at 
other points, in each case computing the moments of the forces about the 
axis A. 

In each case compare the sum of the moments tending to produce clock¬ 
wise rotation with the moment tending to produce counter-clockwise ro¬ 
tation. 


3. When your model bridge is in equilibrium, how does the sum 
of the moments of the forces tending to produce clockwise rotation 
about the point A compare with the sum of the moments of the 
forces tending to produce counter-clockwise rotation about the 
same point? 

4. When A is regarded as the axis, what is the moment of F i 
about this point? Why, then, is this moment not included in either 
of the above sums of moments? 


Laboratory Manual of Physics 


17 


Since we arbitrarily chose A as the axis, we could just as well have 
chosen D, or, for that matter, any point on the meter stick. 

5. What is the moment of F 2 about D as an axis. Of Li about 5? 

6. What is the advantage of selecting A, B, C or D as the axis, 
instead of some other point? 

7. If, in Ques. 2, the automobile is standing 15 ft. from one end 
of the bridge, what is the force exerted by each support? (Re¬ 
gard the bridge as uniform, in which case its entire weight may be 
considered as concentrated at its middle point.) 

Suggested form of record 

A 


Zero reading of balance at P --—g. 

Zero reading of balance at $- g- 

Load In _g; load L 2 -g. 


Trial 

Ft 

in grams 

F, 

in grams 

Ft + Ft 
in grams 

In + Lt 
in grams 

1 





9 





3 






B 

Lever arm of force F 2 , -cm. 



Trial 1 

Trial 2 

Trial 3 

Force Fs in grams 




Lever arm of In, AB, in cm. 




Lever arm of L 2 , AG, in cm. 




Moment of In, L± X AB 


' 


Moment of L 2 , L 2 X AC 




Sum of clockwise moments 




Counter-clockwise moment, F 2 X AD 






















































7. CONCURRENT FORCES 


A child sitting in a hammock is held in equilibrium by three concur¬ 
rent forces. Two of these forces are due to the tension in the hammock 
ropes and the third to the gravitational pull of the earth on the child. 

Imagine the forces due to the two hammock ropes replaced by a sin¬ 
gle force producing the same result, that is, keeping the child from falling. 
This single imaginary force is called the resultant of the two forces due to 
ropes, and the two forces which it replaces are called components. The 
downward force due to the weight of the child is called the equilibrant 
of these components. 


Exp. 7, Part I. Find how the tensions change in two cords support¬ 
ing a load when the angle between the cords is changed. 



Tie the ends of the three cords together 
so that they will not slip. Each cord should 
be about 25 cm long. In the free end of each 
cord make a loop. Hang a load L of about 
1.5 kg from the loop of one cord and slip each 
of the other loops over the hook of a spring 
balance. Suspend the two spring balances 
from the same level, Fig. 9, A and B being 
such a distance apart that the reading on 
each balance is approximately 1500 g. Re¬ 
cord the reading of each balance. 

Repeat with A and B about half as far 
apart, and again with A and B within a few 
centimeters of each other. Arrange data in 
some convenient tabular form. 

1. How does the tension in the sup¬ 
ports change as the angle between 
them is made smaller? 


2. Why will a tightly stretched clothes line often break when 
clothes are hung on it? 

3. Why are the forces in this experiment called concurrent 
forces ? 


4. We know from a previous experiment that when parallel 
forces are in equilibrium, the sum of the forces in one direction is 
equal to the sum of the forces in the opposite direction. From an 
inspection of your data, determine whether this is true when non¬ 
parallel forces are in equilibrium. 

5. How would you arrange the two supporting cords A and B 
so that the sum of the forces exerted by them would exactly 
equal L? 

6. An automobile is stuck in a mudhole. You have a 15 m cable 
attached to the car and 10 m in front of it is a stout post. Which 





Laboratory Manual of Physics 


19 


would you do, and why? (a) pull directly ahead on the cable, (b) 
tie the loose end of the cable to the post and pull sidewise on the 
cable, (c) run the cable around the post and pull on its free end? 

Exp. 7, Fart II. Arrange an apparatus in which the concurrent 
forces are in equilibrium, and find how the resultant of two of these forces 
compares with their equilibrant. 

Use the apparatus of Part I, except that the weight L is replaced by a 
third spring balance, and the whole apparatus is placed in a horizontal 
plane, instead of a vertical plane. 

Fasten the rings of the balances to table clamps placed along the edges 
of the table, Fig. 10, or, if a board with pegs is used, slip the rings of the 
balances over these pegs. Each balance should be stretched to about half 
its full range. 

Place a blank sheet of notebook paper under the cords, with the cen¬ 
ter of the sheet near the point C, and fasten, it down with thumb tacks or 
weights to keep it from slipping. In order to be sure that the balances are 
working freely, make a dot on the paper directly under C, then pull side¬ 
ways on the cords to see if their 
intersection C returns to the same 
place. If it does not, raise the 
balances up slightly to do away 
with 'friction between them and 
the table. 

Now place a wooden block on 
the paper so that its side just 
touches one of the cords but does 
not displace it. With a sharp- 
pointed pencil draw a line on the 
paper along the edge of the block 
and just under the string. Mark 
the directions of the other two 
cords in the same way. 

Read each balance and record 
the tension in each cord at the 
end of the line which represents 
its direction. 

Remove the paper and produce the lines until they meet. Select a 
convenient scale (say 1 cm to represent 150 or 200 g of tension) and 
measure off on each line from C the distance needed to represent the cor¬ 
responding tension. Place at the end of each line an arrowhead to show 
the direction of the force. 

Select any one of the three forces as equilibrant and lay off from C a 
dotted line representing a force equal and opposite to this equilibrant. 
Evidently this dotted line represents the single force which will just bal¬ 
ance the equilibrant. Therefore it must represent the resultant of the two 
remaining forces. 










20 


Laboratory Manual of Physics 


With the two remaining force lines as sides, construct a parallelo¬ 
gram with ruler and compass (see any text-book on plane geometry). 
From C draw the diagonal of this parallelogram. Determine and record 
the magnitude of the force which this diagonal would represent. Com¬ 
pare it with the resultant as given by the dotted line. 

1. How do the resultant and the diagonal of the parallelogram 
compare in direction and in magnitude? How, then, can you find 
the resultant of two concurrent forces? 

2. When three concurrent forces are in equilibrium, how does the 
resultant of two of them compare in magnitude and direction with 
the third? 

3. Make a diagram showing the forces acting on a person sitting 
in a hammock and draw the line representing the resultant of the 
supporting forces. 

4. If the angle ACB were increased, how would this affect the 
length of the diagonal of the parallelogram having AC and BC as 
sides? Does this agree with the results of Part I? 


Special Experiment. Center of gravity. This experiment will show 
that the weight of an object acts as if concentrated at the center of gravity. 

A convenient “object” for use in this experiment is a meter stick 
loaded at one end with a strip of iron or lead. First locate the center of 
gravity of the loaded meter stick by balancing it on the edge of a triangu¬ 
lar block of wood or by suspending it from a cord; balance is obtained by 
adjusting the position of this axis, or pivot, until the meter stick rests in 
a horizontal position. 

Hang a 200 g weight from a point near the light end of the meter 
stick and adjust the position of the pivot until the meter stick again rests 
horizontally. Measure the. distance D of the 200 .g weight from the pivot 
and then calculate the moment, 200 X D, due to the 200 g weight. 

Also measure the distance d of the center of gravity of the loaded 
meter stick from the pivot. Supposing that there is a force W acting down¬ 
ward at the center of gravity, the moment of this force will be W X d. 

Since the meter stick is in equilibrium, the clockwise and counterclock¬ 
wise moments about the pivot are equal; that is, 200 X D W X d. 
Both D and d being known, the value of W can then be obtained. 

A second trial should be made with the 200 g weight placed at some 
other point on the meter stick. Take the average of the values of W ob¬ 
tained from these two trials as the best value of W. 

Remove the 200 g weight from the meter stick and weigh the loaded 
stick on the laboratory balance. 

How does the gravitational force W acting downward on the object 
at its center of gravity compare with the weight of the object? Does the 
weight of an object act as if concentrated at the center of gravity? 


8. THE SIMPLE PENDULUM 


The chief use of the pendulum is to regulate motion in clocks. It is 
also used to measure the acceleration due to gravity g. 

Since pendulums can be made long or short, heavy or light, and with 
a wide swing or a narrow swing, evidently it is important to know how 
these factors affect the rate at which the pendulum beats. 

The length of a simple pendulum is the distance from the support to 
the center of gravity of the bob. When a pendulum swings from one 
end of its arc to the other, it is said to have undergone a single vibration. 
The time required for a single vibration is called the period. The extent 
of swing one side or the other from the central position of rest is called 
the amplitude. 

Exp. 8, Part I. Find how the amplitude, the mass of the bob and 
the length of a pendulum affect its period. 

A. Effect of amplitude. Attach a metal ball or bob to a thread and 
suspend it from a pendulum clamp or nail. Make the length l of this pen¬ 
dulum just 180 cm, the length being measured from the support to the 
center of the spherical bob. 

Fasten a sheet of paper directly behind the bob so that a vertical line, 
which has been ruled on the paper, will indicate the position of the pendu¬ 
lum when it is at rest. 

In measuring the period t of the pendulum, two students should work 
together, one acting as observer and the other as timekeeper. Start the 
pendulum swinging through a small amplitude of about 5 cm and wait un¬ 
til its motion becomes uniform. Then, as the bob passes its position of 
rest, begin to count aloud, zero, one, two, etc., every time the bob passes 
its position of rest. At the signal “zero” the timekeeper observes the time 
on the second hand of a watch, estimating the time to a fifth of a second. 
This will be made easier if the second hand of the watch is observed 
through a magnifying glass. 

When the bob passes its position of rest on the fiftieth count, again 
take the time. 

Keeping the length and amplitude the same, make a second determina¬ 
tion of the time required for fifty vibrations. This second determination 
should agree with the first to within about one second. Take the average 
of the above two determinations and divide it by 50 to get the time of one 
swing, that is, the period t. 

Without changing the length of the pendulum, increase its amplitude 
a few centimeters and again determine the period. Make two trials as 
before. 

Finally, determine the period when the amplitude is very large, say 
1 meter. 

1. Does a small change in amplitude affect the period? 

2. Does a large change in amplitude affect the period? 

[21] 


22 


Laboratory Manual of Physics 


3. Explain why a clock runs too slow when it is first wound up. 

B. Effect of mass of bob. Suspend a second pendulum of exactly 
the same length as the first, but made with a bob of different mass. The 
length of this pendulum should be measured, as always, from the center of 
the bob to the support. Set the two pendulums swinging toother through 
a small amplitude and observe whether one pendulum gains on the other. 

4. What conclusion do you draw as to the effect of the mass of the 
bob on the period? 

C. Effect of length. Shorten the pendulum having a metal bob un¬ 
til it is 45 cm long, which is just one fourth of its original length. Swing 
this pendulum through a small arc and determine its period in the usual 
way. 

Finally, determine the period when the length of this pendulum is 
one ninth of the original length, or 20 cm. 

Compare the periods of the 45 cm and 20 cm pendulums with the 
period which the 180 cm pendulum had when its amplitude was small. 

5. When the pendulum is made one fourth as long, does the period 
become one fourth as much or one half as much? 

6. How does the period change when the length is decreased to 
one ninth of the original length? 

7. Use your data to show that the periods of pendulums are di¬ 
rectly proportional to the square roots of their lengths. 

8. What would have been the period of your pendulum if you had 
made it one sixteenth as long? 

9. Would you shorten or lengthen the pendulum of a clock which 
gains 3 minutes a day? 


Exp. 8, Part II. Use a simple pendulum to find g for your locality. 
The period f of a pendulum is given by the formula, 

y. 


in which l is the length of the pendulum and g the acceleration due to 
gravity. If both members of this equation are squared, we obtain 


f = ^ X 


-f— where * = 9.87. 
g 


Hence, if the length l and the accompanying period t of a simple pen¬ 
dulum are measured at a certain locality, the value of g at that place can 
be computed by means of the above equation. 

To find g for your locality, use a pendulum with a metal bob and of 
length about 180 cm. Swing it through a small amplitude. Measure its 
length l and period t by the method given in A above, making numerous 


trials. 


Compute the average length and period and then find g, using the 
above formula. If l is measured in centimeters and t in seconds, g will be 
obtained in centimeters per second per second. 



Laboratory Manual of Physics 


23 


1. How would the period of your pendulum change if you took it 
to the top of a mountain ? 

2. How long would a simple pendulum need to be to beat seconds 
at your locality? 

3. What information does the formula for the simple pendulum 
give you with regard to, (a) the effect of the mass of the bob on 
the period, (b) the effect of the length of the pendulum on the 
period? 

Suggested form of record 

Part I 


A. Effect of amplitude: 
Length of pendulum, 180 cm. 


Amplitude 
in cm 

Time for 50 vibrations, in sec. 

Period t 
in sec. 

Trial 1 

Trial 2 

Average 

















B. Effect of mass of bob: 


C. Effect of length : 


Length l 

Time 

for 50 vibrations, in sec. 

Period t 


in cm 

Trial 1 

Trial 2 

Average 

in sec. 

Vt 

180 






45 ‘ 






20 







Part II 


Length of pendulum, l - 

Time for 50 vibrations : trial 1__- 
trial 3— 

Average time for 50 vibrations._. 

Period, t ___sec. 

9.87 X ( 

Value of g = -- 


) 

) 3 


.cm. 


sec., trial 2. 
.sec., trial 4. 
_sec. 


sec., 


sec. 


cm per sec. per sec. 





















































9. UNIFORMLY ACCELERATED MOTION 


If a body is being acted on by a single constant force, its velocity will 
increase or decrease at a uniform rate, and it is said to have a uniformly 
accelerated motion. A stone falling in a vacuum is an example. A very 
dense object falling in air has approximately uniform acceleration, since 
the resistance of the air is such a small part of the total force acting on the 
object that this resistance can be neglected. 

Objects fall so quickly through space that it is difficult to study their 
motion. Thus Galileo was led to use a long inclined plane, down which a 
ball will roll more slowly than it would fall vertically. During the War, 
the motion of aerial bombs dropped from aeroplanes was studied by at¬ 
taching lights to the bombs and photographing their paths at night. 

If an object has a uniform acceleration a, the distance s traversed by 
the body in a time t is given by the formula, 

S = i at. 

This formula can be used to measure the height of a tower or the 
length of a steep incline. 

Exp. 9, Part I. Study the motion of an object sliding down an inclined 
plane and verify the formula S = i at 2 . 

Stretch tightly a strong smooth cord from an upper story of a build¬ 
ing to the ground. A weighted glass awning ring or steel ring is to be al¬ 
lowed to slide down this cord, and the slope of the cord is to be such that it 
takes at least 4 seconds for the ring to slide the whole length of the cord. 
Attach a 2.5 cm drilled iron ball or other small weight to the ring. 

Let the ring start sliding from the top and observe the time of fall t 
with a stop watch. Make several observations and compute the average 
time. If the total length of the string is S, 

S — i at 2 , 

where t is the time of fall, which has just been found, and a is the accel¬ 
eration, which is as yet unknown. 

To find a, measure off a distance S' along the lower accessible part of 
the string and repeatedly find the time t' required for the ring to slide 
through this distance. The distance S' should be as long as possible, so as 
to reduce the error in measuring t'. The acceleration a can then be com¬ 
puted by means of the formula, 

S' — | at' 2 . 

Since both a and t are now known, the length S of the string can be 
computed. 

The result should be checked by unfastening the cord and measuring 
its length. 

1. How would the results be affected if the incline were made 

steeper ? Less steep ? 


[24] 


Laboratory Manual of Physics 


25 


2. Show from the formula for S that the distance passed through 
in the first second of fall is equal to one half the acceleration. 

Exp. 9, Part II. Find the height of a water tower, a building or a 
cliff by dropping a stone from its top and measuring the time of fall. 

Measure with a stop watch the time required for a stone to fall from 
the building or other high point. Make several trials and compute the 
average time t. 

The acceleration in this case is g, which may be taken as 980 cm per 
sec. per sec. The height 5 is given by the formula, 

S - i gt\ 

If possible, check your result by measuring with a meter stick the 
length of a weighted cord which will just reach to the ground. 

1. An observer on a high tower measures the time elapsing be¬ 
tween releasing a stone and hearing it hit the ground. Is this the 
time of fall? 

2. A piece of cork and a stone of the same weight are dropped to¬ 
gether from the same height. Will they hit the ground at the same 
time? Explain. 

3. What is the result if two balls of the same size, one of cork 
and the other of lead, are dropped together from the same height ? 


Suggested form of record 

Part I 

Time t to fall whole length of incline : 

Trial 1 ___ sec., trial 2___,_sec.. trial 3_sec. 

Average time, t _sec. 

Known length of cord, S' -cm. 

Time t' to fall known distance S': , 

Trial 1_sec., trial 2 _sec., trial 3 -sec. 

Average time, t’ - sec. 

Acceleration, a — 2/S'/f' 2 = _ cm per see. per sec. 

Length of string, & = % at- =_ cm. or_ in. 


Part II 

(Devise your own form of record.) 












10. BLOCKS AND TACKLE 


Systems of pulleys are used to hoist and lower pianos, safes and other 
heavy objects. The single fixed pulley is often used in raising light loads 
such as awnings and windows. Painters use the arrangement of pulleys 
shown in Fig. 11c to raise and lower their scaffolds. 

Work is not saved by using blocks and tackle. In fact, it requires an 
expenditure of more work to hoist an object with pulleys than it does to 
lift it without them. 

The efficiency of a machine, such as the pulley, is given by the for¬ 
mula, 

useful work accomplished _ W X g/ 

Efficiency = total work“expended ’ 

where W is the useful load, s' the vertical distance is moves, F the force 
required to lift the load, and s the distance this force moves. 

In actual practice this efficiency will always be less than 100 per cent 
because of the presence of friction. Moreover, in pulley systems, the mov- 



Fig. 11. (a) Single fixed pulley, (b) single movable pulley, (c) one fixed and one 

movable pulley. 

able pulleys have to be lifted along with the useful load, and this requires 
an expenditure of work which does not result in useful work being accom¬ 
plished. If there were no friction and the movable pulleys had no weight, 
the useful work accomplished would be equal to the work expended. 

Exp. 10, Part I. Find the efficiencies of several different kinds 
of pulley systems. 

A. Single fixed pulley. Arrange the apparatus as in Fig. 11a, the load 
being about 1 kg. 

T261 






























Laboratory Manual of Physics 


27 


Lift the load W by pulling slowly and uniformly down on the hook of 
the balance, taking its reading as you do so. The force F is the reading 
of the balance plus the weight of the balance. Why? 

Measure the distance s which F must be moved down in order to raise 
W a certain distance s', say 10 cm. 

Tabulate data and calculate in gram-centimeters the work expended 
F X s, and the useful work accomplished W X s', and find the efficiency. 

1. Which is the greater, the expended work or the accomplished 
work, and why? 

2. How do F and W compare? Why, then, is a fixed pulley ever 
used? 

3. Why was it necessary to have the balance moving with a uni¬ 
form speed? 

B. Single movable pulley. Arrange the apparatus as in Fig. lib, 
and follow the same procedure as in Part IA. In this case, however, do 
not add the weight of the balance to the balance reading to get F. Why 
not? 

Notice also that while the total load lifted includes the movable pulley, 
yet the weight of this pulley is not included in W, since in efficiency tests 
you are interested in the useful work accomplished, and not the total work. 

4. How does the efficiency of the single movable pulley compare 
with that of the single fixed.pulley? Explain, bearing in mind that 
there is no increase in friction. 

5. How do F and W compare? Why, then, is the movable pulley 
a useful machine? 

6. The quotient W/F is called the practical mechanical advan¬ 
tage. What information does it give you ? 

C. One fixed and one movable pulley. Arrange the apparatus as in 
Fig. 11c. In this case the useful load W is again the weight hangar and 
weights only, while the force F is the weight of the balance plus its read¬ 
ing, as in Part IA. Follow the same procedure as in Part IA. 

7. How do F and W compare in this case? 

8. How does the practical mechanical advantage W/F of this de¬ 
vice compare with that of the single fixed pulley? 

9. In what way is it a more useful device than the single fixed 
pulley? Than the single movable pulley? 

10. Why does the efficiency differ from that of the single fixed pul¬ 
ley? From that of the single movable pulley? 

11. Show from your data that work is not saved by using a block 
and tackle. 

Exp. 10, Part. II. Test the principle of work. 

We will use a single movable pulley, Fig. lib, to test the principle of 
work. 


28 


Laboratory Manual of Physics 


iSince the principle of work has to do with perfect machines, it will be 
necessary to eliminate the effects of friction. To do this, lift the load W 
by pulling uniformly and slowly up on the ring of the balance, taking the 
reading as you do so. Next lower TP in a similar way, again taking the 
reading. The average of these two readings gives what the force F would 
be if there were no friction. 

Measure the distance s which F must be moved upward in order to 
raise W a certain distance s', say 10 cm. 

Weigh the pulley and add this weight to that of the hanger and 
weights to get the total load W. 

Calculate in gram-centimeters the work expended and the work ac¬ 
complished. 

1. When there is no friction, how does the work expended com¬ 
pare with the work accomplished? 

2. Give the algebraic statement of the principle of work as proved 
by your data. 

3. Give two reasons why the quantity W X s' which you meas¬ 
ured above is not the useful work accomplished? 

4. When friction has been eliminated, W/F is called the theor¬ 
etical mechanical advantage. What is its value for the single mov¬ 
able pulley? 

5. Using the principle of work, show that the theoretical mechan¬ 
ical advantage is also s/s'. 

6. Diagram a system of pulleys having a theoretical mechanical 
advantage of 5. How could this system be changed to give a 
mechanical advantage of 1/5? 

Suggested form of record 

Part I 

Weight of balance_,_g. 


Case 

F 

W 

s 

s' 

F X s 

W x s' 

Efficiency 

Ws'/Fs 

A 








B 








C 









Part II 


F (up)___g; F (down)- 

Weight of pulley— -g. 

Wtark expended = (- «.—-) X ( 

Work accomplished ( 


Total 


.g. Average F. 
load. W _ 

-- ) = — 

^ i 


g-cm. 


) X ( 


g-cm. 































11. FRICTION 


If there were no friction, an engine would not need to exert any pull 
to keep a train running at a uniform speed on a straight level track. On 
the other hand, the engine could not start and stop the train if friction 
were not present. 

Without friction, nails would not hold, the fibers of a rope would not 
hold together, and walking and many other forms of motion would be iim- 
possible. In most machinery, however, it is important to reduce the friction 
as much as possible. This is done by using lubricants, by choosing suit¬ 
able contact surfaces and by substituting rolling contacts for sliding ones. 

Friction is a force which acts in a direction opposite to that in which 
a body is moving. Common forms are sliding, starting, rolling and fluid 
friction. 

If a block of wood is pulled 
at a uniform speed along a level 
board by means of a spring bal¬ 
ance held horizontally, Fig. 12, 

-1 the amount of sliding friction 

between the block and board is 
given by the balance reading. 
The coefficient of sliding friction for these two surfaces can then be ob¬ 
tained by dividing the sliding friction by the force pressing the block 
against the board; this latter is the weight of the block and it should be 
expressed in the same units as the friction. 

Coefficient of friction = I*? 3 ,?- — . 

friction 

The amount of friction depends upon the kinds of surfaces in contact 
and upon other factors, as will be seen in the following experiments. 


OM332)o 


Fig. 12. Sliding friction. 


Exp. 11, Part I. Find how sliding friction depends upon the load and 
upon the area of the surfaces in contact, and also how it compares with 
starting and rolling friction. 

A. Effect of load. Tie a string around a chalk box in such a way 
that it does not touch the bottom of the box and weigh the box by means 
of a spring balance. 

Lay a board on the table, smoothest side up, and place the box on this 
board with a 500 g weight in it. Attach the hook of the spring balance 1 to 
the string, hold the balance horizontally, and use it to drag the box slowly 
but at a uniform speed over the board, Fig. 12. 

If the box is kept moving at a uniform speed, the reading of the 
balance is the sliding friction between the box and board. This reading 
is subject to a correction because the balance is being used in a horizontal 
position, instead of in the vertical position for which it was made. See 
Balances in the appendix. 

Tn place of the spring balance, one may use a weight hanger and weights, fastened to 
the box by a cord hung over a pulley. Add weights to the hanger until the box will movie 
uniformly along the board when started with a slight push. 

[291 












30 


Laboratory Manual of Physics 


It will probably be found easier to read the balance if the board is 
drawn under the box, instead of dragging the box over the board. Since 
the friction will vary a little at different places on the board, several trials 
should be made and the average taken. 

Tabulate results and calculate the coefficient of sliding friction to two 
decimal places. Remember that the load is the weight of the box and its 
contents, expressed in the same units of force as the friction. 

Repeat with the load increased by, say, 300, 500 and then 1000 g. 

1. Does the sliding friction increase with the load? 

2: Within experimental error, does the coefficient of sliding fric¬ 
tion for these two surfaces vary with the load? 

3. What, then, is the exact relation between sliding friction and 
load? 

B. Effect of change in area. Turn the box so that the smoothest 
small end rests on the board, place a 1000 g weight in it, and find the aver¬ 
age value of the sliding friction. Compare this value with that obtained 
with the same load, but with the box resting on its larger base. 

4. How does the friction depend upon the area of the surfaces in 
contact ? 

C. Starting friction. Place about 1000 g in the box and read the 
balance just before the box starts to slide and also while it is moving uni¬ 
formly. 

5. Is starting friction greater than sliding friction? Explain. 

D. Rollmg friction. Without attempting to make careful measure¬ 
ments, compare the sliding friction for a certain load with the friction 
for the same load when two round pencils are placed under the box so 
as to act as rollers. 

6. Describe and explain what you observed. 

7. What pull would an engine have to exert to haul a train weigh¬ 
ing 900 tons at a uniform speed on a level track, if the coefficient 
of rolling friction of the train is 0.009? 

Exp. 11, Part II. Find the coefficients Of sliding friction for various 
surfaces. 

Place a sheet of tin or other material on the board or table and, using 
the method for measuring friction given in Part IA, find the sliding fric¬ 
tion for a load of about 1000 g. Make several trials, compute the average 
value of the friction, and then of the coefficient of sliding friction for the 
two substances. 

Repeat with a sheet of paper, brass, etc., on the board, or with paper 
on both the box and the board. 

1. How would changing the load affect your results? Changing 
the area of contact by turning the box on end? 

2. Explain why polishing a surface reduces friction. 

3. If the coefficient of sliding friction of wood on ice is 0.06, how 


Laboratory Manual of Physics 


31 


heavy a timber can be pushed over ice by a horizontal force of 
of 45 lbs.? 

4. If the friction and load in a certain case were measured in 
pounds of force instead of in grams of force, would the coefficient 
of friction be different? Explain. 

Suggested form of record 

Part I: A 


Weight of box 


Initial reading of balance, horizontal 


Total load 
in grams 

Balance reading in 

grams 

Friction in 
grams 

Coefficient of 
sliding friction 

1 

2 

Q 

O 

Average 























1 



• 



Part I: B, C, and D 

(Devise your own form of record.) 


Part II 



Total load_g. 

Initial reading of balance, horizontal 


Nature of 
surfaces 

Balance reading in grams 

Friction in 
grams 

Coefficient of 
sliding friction 

1 

2 

o 

Average 
















i 



i 

• 






1 




















































































12. TESTING A THERMOMETER 


Thermometers are among the most commonly used scientific instru¬ 
ments. Most people have at least one in the house and one outside. Every¬ 
one wants to know how cold it is in the winter and how hot it is in the 
summer. If you will go into the local store which sells thermometers you 
will find that those placed together in the show case do not indicate the 
same temperature. The variation may be only one or two degrees or it 
may be as much as ten degrees. Even laboratory thermometers may be 
in error several tenths of a degree, and, if used for accurate measure¬ 
ments, they must be tested. Fortunately there are two temperatures at 
which it is possible to make tests with simple apparatus. 

Exp. 12. Test a Centigrade theremometer. 

A. To test the zero point. Fill a tumbler with cracked ice or snow. 
Insert the thermometer until the zero point is just visible. Watch the 

mercury thread and, after it 
has come to rest, read the ther¬ 
mometer. Record this reading 
and then determine the amount 
by which the thermometer 
reads too high or too low. En¬ 
ter this correction in your note 
book, remembering that if the 
reading is below 0°C., the cor¬ 
rection will be positive, and if 
it is above 0°C., the correction 
will be negative. 

1. Where should you 
have your eye when read¬ 
ing a thermometer? 

2. What is meant by the 
statement that the zero 
point of a thermometer has 
a correction of 1.2 °C.? 

B. To test the thermome¬ 
ter at the boiling point. Fill the 
boiler about half full of water and light the burner. While the water is 
heating, place the thermometer in the extension tube with the 100 degree 
mark a few millimeters above the supporting cork. When the water boils, 
watch for the appearance of the mercury, and, after it has ceased to rise, 
take and record the reading. Also read the barometer. 

The amount by which the boiling point varies with barometric pres¬ 
sure 1 has been accurately determined and found to be 0.037 °C. for each 
millimeter change in the barometer. The temperature of steam is 100 °C. 
when the barometric pressure is 760 mm. From this data calculate the true 
temperature of the steam in your boiler. 

Tor an experiment on the effect of pressure on boiling point, see Exp. Add. 4. 

[32] 



Fig. 13. Steam generator. 




































Laboratory Manual of Physics 


33 


In order to determine the effect of exposing the stem of the thermom¬ 
eter to the air, pull the thermometer out of the extension tube until only 
the bulb remains in the steam and note the change in the reading. Tabu¬ 
late all data neatly. 

3. Why must the thermometer not touch the water in the boiler? 

4. Why must the boiler leak steam freely? 

5. Considering the way in which you use a candy thermometer, 
would you test its boiling point with the stem completely immersed 
in the steam? 



Fig. 14. Correction curve for Centigrade thermometer made from the following data : 
Reading of thermometer in ice, —1.45°C.; therefore, correction for a reading of 
—1.45° is +1.45°C. Reading of thermometer in steam, 100°C, the true temper¬ 
ature of the steam being 99.1°C; therefore, correction for a reading of 100.1°G is 
—0.9° C. 

C. To make a correction curve for the thermometer. A knowledge 
of the errors of the thermometer at the fixed points is of little use unless 
a correction curve is drawn which will show the true temperatures for all 
readings of the thermometer. If we assume that the bore of the tube is 
uniform the correction curve will be a straight line. 

Make a correction curve for your thermometer similar to the one 
shown in Fig. 14. See Graphs in the appendix. 

6. At what temperature is your thermometer correct or most 
nearly correct? 


Suggested form of record 

Reading of barometer-mm. 

Reading of thermometer in ice-°C. 

Correction for a reading of- , -°C, is—-°C. 

Calculated temperature of steam-°C. 

Reading of thermometer in steam--- °C. 

Correction for a reading of-—-°C. is-°C. 

Reading of thermometer with stem exposed-°C. 

Difference in reading due to exposed stem 


°C. 















































13. DEW POINT AND RELATIVE HUMIDITY 


The degree of dampness, or humidity, of the atmosphere is of great 
importance not only to the health of individuals but also to many manu¬ 
facturing enterprises of which cotton spinning is one example. Whether 
or not the atmosphere is to be regarded as humid or dry depends not so 
much upon the actual mass of water vapor present as it does upon the 
relative humidity. Relative humidity is defined as the ratio of the mass 
of water vapor actually present in the atmosphere to the mass of vapor 
which would be present if the atmosphere were saturated. 

The dew point is the temperature to which the atmosphere must be 
cooled in order to produce saturation. It is intimately related to relative 
humidity, for the lower the humidity the cooler will the atmosphere have 
to be before the dew point is reached. If either the relative humidity or 
the dew point is known, the other may be obtained easily by the use of 
tables which give the pressure of. saturated vapor at various temperatures. 


Exp. 13, Part I. Determine the dew point and relative humidity oj 
the air in the laboratory. 


A. Experimental determination of the dew point. Observe the tem¬ 
perature of the room. Then choose a brightly polished calorimeter cup 
and place in it tw T o or three centimeters of cold water and a thermometer. 
Add ice, a little at a time, and stir thoroughly (a pencil may be used). 
Watch the surface of the calorimeter ciosely, continue adding ice (and a 
little salt if necessary) until a film of moisture appears on the outside of 
the calorimeter near the bottom, and immediately read the thermometer. 
Next add water gradually and take a reading just as the moisture dis¬ 
appears from the surface of the calorimeter. Now that you have learned 
how to find the dew point, proceed to determine it with greater accuracy, 
making three trials, and taking the average of the six readings as the best 
value of the dew point. Arrange the data neatly in tabular form. 

1. Why must one be careful not to breathe on the calorimeter? 

2. Under what conditions is one likely to need to add a little salt? 

B. Calculation of relative humidity from the dew point. When the 
dew point and the temperature of the atmosphere are known it is an easy 
matter to find the relative humidity, using the Vapor Pressure table under 
Humidity in the appendix. First look in the table opposite the temperature 
of the room, say 22 °C.; you find that the pressure of saturated vapor is 
19.6 mm. of mercury. Then look opposite the dew point temperature, say 
11 °C., and you find the corresponding pressure to be 9.8 mm. Since the 
actual amount of moisture in the atmosphere is proportional to the vapor 
pressure, the relative humidity is given by the formula, 


Relative humidity — 


actual pressure 


possible pressure 

and in the case which we have considered the relative humidity will be 
9.8/19.6 or 50 per cent. 




Laboratory Manual of Physics 


35 


You will note that there is a third column in the vapor pressure table 
which gives the mass of saturated water vapor per cubic meter, at various 
temperatures. These masses are, of course, proportional to the vapor 
pressures, and the ratio of the mass per cubic meter present, to the mass 
which ivould be present were the atmosphere saturated, is the relative hu¬ 
midity. 

3. Show that the relative humidity is the same whether com¬ 
puted from a pressure table or from a table giving the weight of 
water vapor per cubic meter. 

4. Considering that the relative humidity should be from 50 to 
60 per cent, do you find the humidity of your school room satisfac¬ 
tory? If not, how can the condition be remedied? 


Exp. 13, Part II. Determine the relative humidity of the atmos¬ 
phere, using a wet and dry bulb thermometer l 

Read and record the temperature of the atmosphere in the room. 
Then wrap the bulb of the thermometer with a bit of cotton gauze or 
cheesecloth, to form a wick extending one or two inches below the bulb. 
Suspend the thermometer with the end of the wick in a tumbler of water 
at room temperature. Fan the thermometer vigorously. When the mer¬ 
cury reaches the lowest point, read and record the temperature. The 
dryer the atmosphere the more rapid is the evaporation and the lower the 
reading of the thermometer. The relation is not a simple one and tables 
have been prepared to give the relative humidity when the dry and wet- 
bulb readings are known. 

A table giving Relative Humidity from ivet and dry-bulb thermometer 
readings will be found under Humidity in the appendix. 

1. Is the humidity in your home sufficiently high? What are 
you or your parents going to do about it? 

2. Give a brief statement of a few reasons why humidity is of 
considerable importance. 

Suggested form of record 

Temperature of room -°C. 

Temperature at which dew forms: (1)-°C., (2)-°C., (3)-°C. 

Temperature at which dew disappears: (1)---°C., (2)-°C., (3)-°C. 

Dew point-°C. 


Pressure of saturated vapor at dew point-mm. 

Pressure of saturated vapor at temperature of room-mm. 

actual pressure 

Relative humidity = —-- = - = -P er cent - 

possible pressure 


“This is an excellent experiment for the student to perform at home, as the humidity of 
the house is such an important factor in the health of a family. A household thermometer 
can be used but a laboratory thermometer is more convenient. The table in the appendix 
is for Centigrade readings and. if a Fahrenheit thermometer is used, either convert to Centi¬ 
grade or use a Fahrenheit table. 
















14. COEFFICIENT OF LINEAR EXPANSION 


Many useful appliances such as thermometers, thermostats, etc., de¬ 
pend upon the fact that substances expand with increase of temperature. 
On the other hand, this property of expansion may be a disadvantage, as 
is shown by the fact that expansion joints must be placed in railroad 
tracks, steam lines, bridges, etc. Different substances expand different 
amounts and in many cases account must be taken of this fact. For ex¬ 
ample, the wires leading the current through the glass of an electric light 
bulb were for a long time made of the expensive metal platinum because 
no other metal or alloy could be found with the same expansion as glass. 
It was only after years of scientific research that a substitute was devised. 

A large amount of information about the expansion of various sub¬ 
stances is published in handbooks, but it is often necessary in industrial 
and other scientific laboratories to test the expansion of samples of ma¬ 
terials. In the present experiment we learn one method of making this 
important measurement. 

In order to have a standard of comparison it is necessary to express 
expansions in terms of a given length of material and a given change of 
temperature. The coefficient of linear expansion of a substance is the 
amount of expansion which a unit length of the material experiences when 
heated through one degree. This is equal to the ratio of the increase of 
length, per degree rise in temperature, to the total length. 

In order to determine this coefficient it is necessary to heat a given 
length l of the substance through a known change in temperature t 2 —ti, 
and measure the increase in length i. Then, 


Coefficient of linear expansion = 


increase in length per degree 
total length 



Exp. 14. Measure the linear expansion of brass. 

Arrange the apparatus as indicated in Fig. 15, handling the tube as 
little as possible so that its temperature fi will be that of the room. The 



temperature of the steam t 2 may be calculated (see Exp. 12) or measured 
by a thermometer inserted in the end of the tube. 

The increase of length i is so small that a multiplying device, consist- 


136] 














Laboratory Manual of Physics 


37 


ing of the roller and pointer, mlust be used. Care should be taken not 
to move or jar the apparatus after the steam is applied. 

When all is ready read the position Vi of the end of the pointer, read¬ 
ing to 0.1 mm. Then light the burner and wait until steam issues freely 
from the tube. When the reading of the pointer becomes constant, ob¬ 
serve its position p 2 . The length of the tube l from the shoulder to the 
roller should then be measured, and also the diameter of the roller. As 
this is small,, a micrometer caliper should be used and read to .001 cm. 
All measurements of length should be expressed in the same units, prefer¬ 
ably centimeters. 

The increase of length i of the tube is determined from the diameter 
of the roller d, the length of the pointer r and the distance p 2 — Pi through 
which the end of the pointer moves on the mirror scale. As the tube 
moves the distance i, Fig. 16, the roller is tipped through the same angles 
as the pointer. By similar triangles, 


i P 2 — Pi 

d r 

and 

i _ (P a — Pi ) d ' 
r 

1. Why is it necessary to read the diameter of the roller to .001 
cm and the length of the tube to only 1 mm? 

2. What would be the difference in the value of the coefficient 
of expansion if a Fahrenheit thermometer were used and the co¬ 
efficient were expressed in terms of a Fahrenheit degree? 

3. What would be the difference in the value of the coefficient if 
English units of length had been used instead of the metric units? 

Suggested form of record 

Observed Data Calculated Data 



Length of tube, l -cm. 

Initial temperature, ti _°C. 

Final temperature, t 2 _°C. 

First scale reading, p\ -cm. 

Second scale reading, p- _cm. 

Diameter of roller, d _cm. 

Length of pointer, r _cm. 


Change of temperature, 

U — ti = __°C 

Increase in length, 

(Pa — Pi)d 

i == -- -- < 

r 

Coefficient of expansion of brass, 

k = - 1 2 3 - = - 

(U — U)l 

Accepted value of lc __ 

Per cent of difference 




















15. SPECIFIC HEAT 


The specific heat of a substance is a measure of its ability to store 
heat The high specific heat of water makes it an ideal substance for hot 
water bottles.” The fact that water is a liquid and that it changes to 
steam at high temperatures unfits it for use in fireless cookers but a solid 
substance such as soapstone, which has a comparatively high specific heat, 
is well adapted to this purpose. Substances vary greatly m specific heat, 
as will be found in this experiment. 


This experiment depends upon the principle that a hot body immersed 
in a cup of water will give up its heat to the water and the cup, and that 
the heat lost by the hot body will exactly equal the heat 
gained by the water and cup. This is true only if no 
heat is lost and one of the greatest difficulties in heat ex¬ 
periments is to prevent heat from escaping from the in¬ 
ner cup of the calorimeter or entering it from the out¬ 
side. Since air is a poor conductor of heat, the calori¬ 
meter which you are using is made in two parts with a 
layer of air between them. 

In order to allow for the.heat absorbed or given up 
by the inner cup, it is necessary to know its mass and 
specific heat. Calorimeter cups are usually made of 
brass, specific heat 0.1, or of aluminum, specific heat 0.2. 

Since the specific heat of a substance is the amount 
of heat required to raise the temperature of one gram 



Fig. 17. Calorimeter. 


of the substance 1°C., the heat taken in or given out by a sample of the 
substance is equal to the mass of the sample times the change in tempera¬ 
ture times the specific heat of the substance of which the sample is com¬ 
posed. We may write, 

Heat lost by substance — heat gained by water 4- heat gained by 
calorimeter, 


or 

M a (U — t mix ) S ~ M w (£ mix — t w )l + M c (t m ,* — U) S c , 

in which M s is the mass of the sample, t s the temperature of the sample 
when hot, t mix the temperature of the mixture, S the unknown specific heat 
of the substance, M w the mass of the water, t w the initial temperature of 
the water, M c the mass of the calorimeter, and S c the specific heat of the 
calorimeter. 

No experiment illustrates, better than this, the necessity of laying 
out a plan for one’s experimental work and of exercising skill in the 
manipulations. Remember that you will not always have the directions 
of a text-book and, that you should try to develop in yourself the ability 
to outline the procedure. For example, the weight of water may be de¬ 
termined easily and accurately if the calorimeter is weighed first without 
water and then with the water in it, the difference in the two weights be¬ 


tas] 













Laboratory Manual of Physics 


39 


ing the weight of the water alone. This is much better than to measure 
or weigh the water in some other vessel as, in either case, some water will 
be left behind. A precaution which must be taken is to have neither too 
much nor too little water. If you have too much, the temperature in¬ 
crease will be small. Another precaution against error is to make the 
initial temperature of the water about as many degrees below room tem¬ 
perature as you believe the mixture will be above room temperature. 

1. How does it happen that special precautions must be taken to 
prevent loss of heat? 

2. Explain in your own words the reason why it is desirable, 
in calorimeter measurements, to start with the calorimeter as 
much below room temperature as you expect it to be above room 
temperature at the end of the experiment. 

Exp. 15. Measure and compare the specific heats of several sub¬ 
stances. 

Substances such as lead, aluminum, copper, glass, and steel are to 
be tested, each group of students working with from one to three sub¬ 
stances. The following instructions are written for lead in the form of 
shot. First start the boiler so that the water may be heating; then weigh 
out enough shot to fill the dipper nearly to the top. Carefully push a 
thermometer into the shot and put the dipper in the boiler. 

While the shot is heating, weigh the inner calorimeter cup, put in it 
the proper amount of water (the less the better if enough to cover the 
shot), and then weigh the calorimeter and contents. If ice or snow is 
available, add it to the water until its temperature is reduced to about 
10° C. below room temperature. 

When the boiler has been generating steam for several minutes ob¬ 
serve the temperature of the shot t s , and of the water in the calorimeter t w . 
Quickly pour the shot into the calorimeter. Stir the mixture and read its 
temperature with a thermometer, recording the maximum, reading t mix . 
Arrange the data in neat tabular form and repeat the experiment for as 
many substances as you have time. 

If other students are working with other substances, record along 
with your results, the names of the other students, the substances tested, 
and the value of S obtained by them. This gives you a small specific heat’ 
table determined in your laboratory, which may be compared with tables 
in the books. 

3. Why is greater accuracy secured by using a small amount of 
water in the calorimeter cup? 

4. Why is it more necessary to stir glass beads than it is to stir 
copper pellets when they are being heated? Why is it more essen¬ 
tial to have glass in pellet form than copper or aluminum? 


40 


Laboratory Manual of Physics 


Suggested form of record 


Substance 

Mass of 
Sub¬ 
stance 

M s 

Mass of 
calori¬ 
meter 

M 

c 

Mass of 
calori¬ 
meter 
and 
water 

Mass of 
water 

M 

w 

Temp, of 
subst. 

K 

Temp, of 
water 

K 

Temp, of 
mixture 

^mix 

Specific 
heat of 
subst. 

8 

Value of 

S from 
table 




















































Special Experiment. The household thermometer. This experiment 
is for a student who wishes to test one of the thermometers used at home. 
Determine the freezing point as in Exp. 12, A. Then place the household 
thermometer and the laboratory thermometer, which you have already 
tested, in a beaker, or deep vessel, filled with a mixture of ice and water. 
Heat the water gradually and, after each increase of temperature of about 
ten degrees on the household thermometer, take readings of both instru¬ 
ments. During the time that each reading is being taken, the flame should 
be turned down or removed and the water thoroughly stirred, so that both 
thermometers may be at the same temperature and not changing in tem¬ 
perature too rapidly. 

Determine the correction for each reading of the household thermome¬ 
ter, plot this data on graph paper and construct a correction curve by con¬ 
necting adjacent points with straight lines. Why is it important that both 
thermometers be immersed in the water as high as the mercury column? 


Special Experiment. The vacuum bottle. Place in a vacuum or 
Thermos bottle, a known weight of ice-cold water, take its temperature, 
allow the bottle to stand closed for a known period of time, say 10 hours, 
and again take the temperature of the water. Calculate the number of 
calories of heat which enter the bottle per hour. 

Repeat the experiment with a known weight of hot water and cal¬ 
culate the number of calories of heat lost by the bottle per hour. 

Does your vacuum bottle keep water cool better than it keeps it warm ? 
Explain. 

Describe the shape and the nature and appearance of the materials 
of which the vacuum bottle is made. Make a diagram showing its con¬ 
struction. 





































16. THE MECHANICAL EQUIVALENT OF HEAT 


«" 


The measurement of the mechanical equivalent of heat is of interest 
for reasons quite different from* those which lead us to test thermometers 
or to determine the specific, heat of a substance. In an industrial laboratory 
one might be called upon at any time to test a thermometer or to measure 
specific heat, but never to measure the mechanical equivalent of heat. This 
experiment interests us because it enables us to repeat, in a crude way, one 
of the important experiments in the history of physics, for not only did it 
open the way for a better understanding of heat phenomena, but it also 
lead to the great principle of the conservation of energy. 

In this experiment, you will have an opportunity to show that mechan¬ 
ical energy changes into heat energy and you will be able to compare your 
numerical result with the value which has been obtained by the most 
accurate methods. 

When a body is raised to a certain distance, a definite amount of 
work is done against gravity and the body gains potential energy. If the 
body falls back through this same distance, the potential energy changes 
to kinetic energy. When the body is stopped there is no more motion 
and no kinetic energy; the energy reappears in the form of heat. In or¬ 
der to find the mechanical equivalent of heat, or the number of calories 
produced by one gram-meter of mechanical work, it is necessary to 
measure the mechanical energy which disappears and the heat energy 
which is produced. The energy of a falling body is equal to its mass X 
the height of fall, while the heat produced, if it can be confined to the 
body, is equal to the mass of the body X its specific heat X the increase 
in temperature. 

1. Does the stirring of ice cream in the freezing process heat the 

cream? If so, why is a stirrer used? 

Exp. 16. Determine the mechanical equivalent of heat. 

In this experiment lead shot are caused to fall from one end of a 
long cardboard tube to the other. In falling once through the length of the 



Fig. 18. Simple apparatus for determining the mechanical equivalent of heat. 


tube the shot will generate but little heat. If it falls a number of times, 
the increase of temperature will be appreciable. The tube should be 
grasped in both hands with the hands far enough away from the ends 
so that the heat from the hands will not pass to the shot. Great care 
must be taken to see that the shot falls the length of the tube, no more, 
no less. Practice handling the tube before using the shot. 

Weigh out about 2 kg of lead shot, measure its temperature t u and 
pour it in the tube. Insert the cork; plug the hole in the cork with a 
pencil, or a piece of wood, and tie the cork securely, so that it cannot pos- 


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42 


Laboratory Manual of Physics 


sibly come out. Reverse the tube 80 to 100 times, keeping accurate count; 
the last time have the cork at the bottom. 

Rest the end of the tube near the edge of the table, inclined suffi¬ 
ciently so that the shot will not run out when the plug is withdrawn. 
Take out the plug and insert the thermometer. Tip the tube up so that 
the shot forms a compact mass around the bulb of the thermometer and 
observe its maximum reading t 2 . Next reverse the tube, remove the ther¬ 
mometer and cork, and measure the distance h from the shot to the point 
formerly occupied by the end of the cork. This distance is the average 
distance through which the shot fell at each reversal. 

The mechanical equivalent of heat may be expressed in different 
kinds of units. In this experiment we will calculate the number of gram- 
meters of mechanical energy which must pass over into heat energy in 
order to produce one calory. The number of calories of heat produced is 
the mass of the lead M X the specific heat of lead, 0.03. X the 
change in temperature, t 2 — t x . The work done, expressed in gram- 
meters, is equal to the mass of the shot (grams) . X the height of fall 
(meters) X the number of reversals (N ). Then, since the mass cancels 
out, the mechanical equivalent of heat J is given by 

N X L 
(t 2 — U) .03 

2. If the accepted value for the mechanical equivalent of heat is 
427 gram-meters per calory, how high must a waterfall be for 
the water to become 1°C. warmer at the bottom than at the top? 

3. If you were to repeat this experiment how would you change 
the instruments or procedure in order to get better results? 

4. Explain the reasons why a worker in an industrial laboratory 
would never be called upon to measure the mechanical equivalent 
of heat. 


Suggested form of record 

Mass of shot, approximate-£. 

Initial temperature, h -°C. 

Final temperature, U --°C- 

Number of reversals, N - 

Height of fall, L - m. 

Mechanical equivalent of heat_gram-meters/calory. 

Accepted value -_gram-meters/calor.v. 


Per cent of error 











17. CHANGE OF STATE 

A curious and interesting condition can be brought about if a liquid 
is kept very quiet while being cooled, for under these circumstances the 
liquid will cool below the freezing point. This phenomenon is called 
undercooling. The moment that the liquid begins to solidify, the tem¬ 
perature rises to the freezing point and remains there until all the liquid 
is solidified. After solidification is complete the solid gradually cools off 
and its temperature goes down. Acetamide/sodium thiosulfate (hypo), 
water, and many other substances exhibit the phenomenon of under 
cooling. 


Exp. 17. 


Obtain the cooling curve for acetamide. 


Oo 


Support a test tube half filled with crystals of acetamide in a beaker 
of water on a ring stand, Fig. 19. When the acetamide is melted insert 
the thermometer, making sure that the 
acetamide does not obscure the 50 degree 
mark. Cut off the heat when the temper¬ 
ature reaches 90°C. and remove the 
beaker. The tube must now be allowed 
to cool and must not be disturbed in the 
slightest. 

Begin immediately to observe tem¬ 
peratures each half minute and keep a 
careful record. Indicate the time at 
which solidification begins and continue to 
make readings at half-minute intervals un¬ 
til solidification is complete, then at min¬ 
ute intervals until a temperature of about 
50° is reached. 

Record the data neatly in two col¬ 
umns, the first column showing the times 
(to the half minute) as indicated by the 
w'atch, and the second column the corre¬ 
sponding temperature. 

Plot a curve', similar to Fig. 

20, which will show the character¬ 
istic cooling curve for acetamide. 

1. What part of the curve 
indicates under¬ 
cooling? 

2. How do you 





r 





Fig. 19. 


T/me 

Fig. 20. Typical cooling curve. 


1 See Graphs in the appendix. 


account for the horizontal 
part of the curve? 

3. What is the freezing point of aceta¬ 
mide? 

4. Where did the heat come from when 
the acetamide “heated up”? 


[ 43 ] 



























18. HEAT OF FUSION OF ICE 


The heat of fusion of a substance is the number of calories of heat 
required to melt one gram of the substance. The determination of the 
heat of fusion of ice has, of course, been done very accurately. Never¬ 
theless, it is an interesting experiment to perform and it furnishes a test 
of one’s skill as an experimenter, for, even with crude apparatus, a careful 
person can secure a result differing from the accepted value only by two 
or three per cent. 

This experiment is closely related to Exp. 17 and, if you have not 
performed that experiment, you should read the introduction to it. In 
Exp. 17 we studied the temperature changes which occurred when ace¬ 
tamide changed to a solid while in the present experiment we will measure 
the heat taken up by ice when it melts. Why is this the same as the heat 
given up by water in freezing? 

The heat of fusion of ice is measured by placing a piece of dry icq in 
some water contained in a calorimeter cup. The water and calorimeter 
cup furnish heat to the ice, causing it to melt. 

Heat lost by calorimeter and water — heat gained by ice. 

Notice that two things happen to the ice, each of which requires heat. 
First, the ice melts and then the resulting water is heated from 0° C. to 
the final temperature of the mixture, C ix . The equation may be written 
as follows: 

M w (ti — t mix ) -f M c Hi — t m} J S e m M\ X L + M; (t mix — 0° C .) 
in which L is the heat of fusion of ice and the other symbols are self-ex¬ 
planatory. 

Errors will be reduced if the initial temperature of the water is 
as much above room temperature as the final temperature of the mixture 
f raix is below. Why? 

Exp. 18. Measure the heat of fusion of ice. 

In making the weighings in this experiment great care should be 
taken. First obtain the mass of the inner calorimeter cup M c and then 
fill it two-thirds full of water which has been heated to about 25° C. above 
room temperature. Then weigh the calorimeter cup and water accurate¬ 
ly, M c + w . Place the inner cup in the outer calorimeter cup; stir and 
observe the temperature of the water, which should now be about 10° or 
15° C. above the temperature of the room. In the meantime another stu¬ 
dent should have prepared a piece of ice about the size of a hen’s egg. As 
the temperature of the water is being observed, this piece of ice should be 
dried carefully and thoroughly with a piece of paper towel or other ab¬ 
sorbent material and slipped immediately into the calorimeter cup with¬ 
out splashing. Stir gently and observe the minimum temperature, t mix , 
just as the last ice melts. If this temperature is not about as many de¬ 
grees below room temperature as the water was above, repeat the ex¬ 
periment, making such changes in quantities or temperatures as may be 


[ 44 ] 


Laboratory Manual of Physics 


45 


necessary. Remember, other things being equal, that the larger the piece 
of ice the more accurate the result. Why? 

Remove the thermometer, taking care that no water is lost, and make 
a final weighing of the calorimeter cup and contents, M c + w + f . 

1. What is the temperature of the ice just as you slip it into the 
water ? 

2. What is the temperature of water dripping from melting ice? 

3. Explain why it is far more necessary to get the mass of the 
ice accurately than the mass of the water or calorimeter. Explain 
how you obtained the mass of the ice? 

4. Heat is required to change ice at 0°C. into water at 0°C; 


what becomes of this heat? 

Suggested form of record. 

Weight of calorimeter, water, and ice, M . _ g. 

0 W + 1 

Weight of calorimeter and water, M _i_g. 

Weight of calorimeter, M c _g. 

Weight of ice, M c w , . — M c t w __g. 

Weight of water, M c ' w — M _ g. 

Temperature of water, f _ _ g. 

Temperature of mixture, # mix _ _ g. 


Special Experiment. Absorbing and radiating surfaces. Secure 
two bright tin cans of the same size and blacken one of them with lamp¬ 
black and shellac or by holding it in a smoky flame. Fit each of the cans 
with a one-hole stopper containing a thermometer. Fill both cans with 
cold water of the same temperature, close them tightly with the stoppers, 
and suspend them by cords in the sunlight. Read and record the ther¬ 
mometers at known intervals of time for a period of one hour. Which 
kind of surface do you find to be the better absorber of radiant energy? 

To find which surface radiates energy more rapidly, fill the cans with 
hot water of the same temperature, hang them in a cool place and make 
temperature observations at known intervals of time for a period of one 
hour- 

Why are the walls of thermos bottles silvered? What kind of paint 
would you choose for a refrigerator? 










19. SPEED OF SOUND 


The speed of sound in air can be measured by noting the time be¬ 
tween the flash and the report of a gun fired at a known distance away. 
It can also be found by striking a hammer against a board at a known dis¬ 
tance from the flat wall of a large building, and finding the time that 
elapses between striking the board and hearing the echo. In this case, 
twice the distance to the wall, divided by the time, gives the speed of 
sound. 

Both of these methods give only approximate results, although they 
serve as excellent illustrations of the speed of sound. They involve not 
only errors of observation, but errors due to changes in the magnitude 
and direction of the wind and to local differences in temperature. 

Sound consists of waves in an elastic substance. It will not pass 
through a vacuum, as the presence of an elastic substance is essential. 
The more elastic a substance is, the faster does sound travel through it. 
Thus it travels at a high speed in steel but at a very low speed in rubber. 

The less dense a particular substance becomes, the faster does sound 
travel through it. The speed of sound in air increases with a rise in 
temperature, chiefly because heat causes air to expand, and thus de¬ 
creases its density. 

Exp. 19, Part I. Measure the speed of sound in air. 

Two groups of observers station themselves 500 m or more apart and 
with an unobstructed view between them. The distance between the two 
stations is measured with a long cord or rod of known length. The tem¬ 
perature of the air is also read and recorded. 

A member of the group at one station gives a prearranged warning 
signal and then fires a blank cartridge. At the other station an observer 
measures the time between the flash of the pistol and its sound. This 
procedure is repeated four or five times. 

In order to eliminate effects due to the wind, the procedures at the 
two stations are interchanged, so that the direction of the sound is re¬ 
versed. Repeated trials are again made. 

The average of all these measurements is taken as the time it takes 
sound to travel the measured distance. The speed of sound in air at the 
observed temperature can then be calculated and this should be recorded 
in meters per second. 

The speed of sound in air at 0° C. should also be calculated. Remem¬ 
ber that the speed increases about 0.6 meters per second for a rise in 
temperature of 1° C. 

Arrange your observed and computed results in tabular form. 

1. Why not make allowance for the time required for the flash of 

the pistol to reach the observer? 

2. Explain why interchanging the procedure at the two stations 

tends to eliminate the effects of the wind. 


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Laboratory Manual of Physics 


47 


3. If a cannon is fired at 5:15 p. m. on a calm day, at what time 
will the report be heard at a point 30 km away, the temperature 
of the air being 22° €.? 

Exp. 19, Part II. Compare roughly the speeds of sound in air and in 
steel. 

The experiment is performed on a straight stretch of railroad track. 
One member of the class goes some distance down the track and strikes 
the rail with a hammer or a piece of iron while the remainder listen with 
their ears placed close to the rhil. 

1. Explain what you observed. 

2. If a sound were heard through a steel rail in 0.1 sec., and then 
through the air 1.3 sec. later, and the temperature was 20° C., what 
was the speed of sound in the rail ? 


Special Experiment. Interference of sound waves. Sound a tuning 
fork of frequency 256, or thereabouts, and hold it close to the ear. Turn 
the fork slowly about its stem as an axis until you find positions of the fork 
where the sound is loudest and positions where it is faintest. 

In what positions relative to your ear are the prongs .when the sound 
is faintest? If you have difficulty in making the observations necessary to 
answer this question, hold the vibrating fork near the mouth of a resonance 
tube (Fig. 21), adjust the length of the tube until resonance is obtained 
(see Exp. 20), and then rotate the fork about its stem as an axis until you 
find the positions of the prong for which the sound becomes faintest. 

The two prongs of a tuning fork vibrate with the same frequency, 
but in opposite directions. When you hold a vibrating fork near the ear, 
with the two prongs in the straight line joining the fork and the ear, the 
prong closest to you moves toward you (producing a condensation on the 
side nearer to you) ; at the same time the prong farthest from you moves 
away from you (producing a rarefaction on the side nearer to you). The 
condensation and rarefaction do not reach your ear at exactly the same 
time. If you now rotate the fork slightly, the farther prong is brought 
nearer to your ear, while the nearer prong is moved farther away. By 
properly adjusting the distances of the prongs from the ear, the condensa¬ 
tion from the one prong and the rarefaction from the other will reach 
your ear at the same time and, interference will result; the two sounds 
will add to produce silence. 

To test this explanation, find a position for the vibrating fork where 
the sound is silent, or nearly so, and then completely cover one of the 
prongs, without touching it, with a small paper tube. Do you again hear 
the sound? Explain. 


20. WAVE LENGTH OF THE NOTE OF A TUNING FORK 


If a vibrating tuning fork is held over the mouth of a pipe closed at 
the other end with a movable piston, Fig.. 21, a very marked reinforce¬ 
ment of the sound will occur when the air column is made about one- 
fourth as long as the wave length produced by the fork. If the length 
of the air column is then increased by a distance which is just one-half ot 
the wave length of the fork, maximum reenforcement will again occur. 
Hence, if Sr denotes the shortest length of air column for maximum reen¬ 
forcement, and s 2 the next length, the wave length l of the work is given 
by the relation, 

1 — 2 ( 82 — 3 ,). 


Fig. 21. Horizontal resonance tube with sliding piston. 


A second way of find¬ 
ing the wave length l of 
the fork is by means of the 
formula, 



where v is the speed of sound in air at the given temperature and n is the 
number of vibrations per second made by the fork. 



=EL™ 


Exp. 20. Find the length of the sound wave emit¬ 
ted by a vibrating tuning fork. 

Set the tuning fork into vibration by striking it 
once on a flat cork. Quickly hold it in front of the 
mouth m of the.tube, with one of the prongs next to the 
opening, Fig. 21 or Fig 22. At the same time pull 
the piston slowly away from this end until a posi¬ 
tion is found at which the sound is loudest. Mark 
the position of the front of the piston with a rubber 
band and then try the effect of moving the piston back 
and forth past this point until the best place is located. 

When found, measure the distance from the mouth m 
to the rubber band, calling this distance Sj. Now re¬ 
peat the whole procedure and take the average as the 
correct length s x of the air column for the maximum 
reenforcement. 

Find a second length of air column giving maxi¬ 
mum reenforcement. This column will be about three 
times as long as the first one. As before obtain the 
correct length by making several trials and computing Fig. 22. Vertical form 
the average length s 2 . JfcSUd 

The length of the waves set up by the fork raising and lowering the 
can now be calculated by the formula, l 2 (s 2 — Si). water< 

Hold a thermometer in the tube and find the temperature of the air. 
Also read the vibration number n marked on the fork. Then calculate 

[48] 






















Laboratory Manual of Physics 


49 


the wave length l by means of the formula l = v/n and compare this value 
with that obtained with the resonance tube. The speed of sound in air 
may be taken at 3'32 m per sec. at 0°C., with an increase of 0.6 m for 
each degree rise of temperature. 

The experiment should be repeated with a tuning fork of another 
frequency. 

1. How could a resonance tube be used to find the vibration rate 
of a tuning fork? 

2. What effect has a drop in temperature on the pitch of an 
organ pipe? 

Suggested form of record 

Frequency of fork, n ___ 

Temperature of air_°C. 

Length of air column for first resonance, 8i: 

Trial 1,__cm; trial 2_cm; Average _ cm. 

Length of air column for second resonance, s 2 : 

Trial 1, _ cm ; trial 2_cm ; Average - cm. 

Wave length, l — 2(s 2 — Si) = -cm. 

Wave length, l = vJn _cm. 


(Make a similar table for each additional fork tested.) 


Special Experiment. Another method for calculating wave length. 
The shortest length of air column for maximum reenforcement Si is 
approximately one-fourth wave length. It is not exactly one-fourth wave 
length because the mouth m of the tube acts as if it extended farther out 
than it actually does. It has been found that if about four-tenths of the 
internal diameter d of the tube is added to the length of the air column s u 
the resulting length, s x -f- 0.4 d, will be almost exactly one-fourth wave 
length. 

Measure the internal diameter d of the tube and the shortest air col¬ 
umn for maximum,resonance Si with a certain tuning fork, if this has not 
already been done. Using these data, compute the wave length l of the 
fork and compare this value with those obtained in Exp. 20 with the same 
fork. Which is the better method of measuring the wave length, and why? 












21. MAGNETIC FIELDS 


The region about a magnet in which its effect can be detected is 
called the magnetic field of the magnet. If a magnetic pole is placed in 
such a field it will be acted upon by a force. The direction and magnitude 
of this force will not be the same in all parts of the field. 

A small compass placed in a magnetic field points in the direction 
of the magnetic field. If the compass is placed near the N-pole of a mag¬ 
net and then moved a short distance at a time, in the direction its 
north pole points, it will be found that it travels along a line which finally 
ends at the S-pole of the magnet. Such a line is called a magnetic line of 
force. These lines of force show the direction in which the magnetic 
force would act at the various points in the field, if magnetic poles were 
placed at these points. By the direction of a line of force is meant the 
direction in which the N-pole, or north-seeking end, of a compass needle 
points when placed on the line. 

If several magnets are placed in the same region, the magnetic field 
at any point will be the resultant of the fields at that point due to the 
several magnets. This explains why a very small compass must be used 
in tracing lines of force in a field; a large compass needle would be strong 
enough to produce noticeable changes in the field in which it was placed. 
In performing the following experiment, it must be remembered that the 
earth is a magnet and that it is likely to produce slight changes in the 
fields of the magnets being investigated- 

Exp. 21. Make charts of several different types of magnetic fields. 



Fig. 23. Charting a magnetic field. 


A. The field about a single bar 
magnet. Place a sheet of paper on 
a smooth table. With the help of 
a compass, turn the paper so that 
its longer edges are parallel to the 
compass needle. In doing this, be 
sure to have all magnets and mag¬ 
netic materials several meters dis¬ 
tant from the compass. The paper 
must not be moved from this posi¬ 
tion during the experiment. 

Lay a bar magnet on the paper 
as in Fig. 23, trace with a pencil 
the outline of the magnet and mark 
on the drawing the positions of its 
N-pole and S-pole. 

Near, say, the N-pole of the 
magnet make a dot (1), which is to 
serve as a starting point. Place the 
small tracing compass on the paper 
so that the south end of the needle 
is right above this dot, and then 


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Laboratory Manual of Physics 


51 


make a second dot (2) at the other end of the needle. Now move the com¬ 
pass until its south end is at (2) and make a third dot (3) at its north end. 
Continue this series of dots until you reach either the edge of the paper or 
the S-pole of the bar magnet. Draw a smooth curve through this series 
of dots and indicate by arrowheads the direction of this line of force 
which you have drawn. 

Beginning at other points near the same pole of the bar magnet, 
trace several more magnetic lines of force on each side of the magnet. 
Then, starting from the S-pole of the bar magnet, trace additional lines 
from that end. Enough lines should be drawn, and their starting points 
should be so chosen, that the nature of the whole field about the magnet 
can be seen from the drawing. 


B. 



Combinations of 



magnets, (a) Place a fresh sheet of paper on 
the table and with the aid of a compass, ar¬ 
range it as in A, above. Place on the paper 
two bar magnets in line with each other and 
with their unlike poles about 15 cm. apart, 
Fig. 24a. Using the method given in A, 
above, trace with the small compass at least 
six magnetic lines of force leaving each of 
the adjacent like poles. 


(b) Make another chart but with un¬ 
like poles facing each other as in Fig. 24 b. 



(a) 


Fig. 24. 

1 . 


(c) Place a piece of unmagnetized soft 
iron, such as an iron washer, between the un¬ 
like poles, Fig. 24c, and make a chart of this 
field as before. Be sure to have some of the 
Combinations of magnets, lines pass through the iron. 

What is the form of the field between two like poles? Between 



two unlike poles? 


2. Did any of the lines cross each other? 

3. Can you find on your charts any effects due to the earth’s mag¬ 
netic field? 

4. What is the result when a piece of soft iron is placed in a 
magnetic field? 

5. Did you find that the poles of the bar magnet were located 
exactly at the ends? 














22. THE NATURE OF MAGNETISM 

If a piece of unmagnetized iron is held near either end of a com¬ 
pass needle, the needle is attracted by the iron. A substance which at¬ 
tracts both ends of a magnet is a magnetic substance, but it is not a mag¬ 
netized substance. When two substances, both of which are magnetized, 
are brought near each other, the like poles repel and the unlike poles at¬ 
tract each other. 

According to the theory of molecular magnets, magnetic substances 
are made up of molecules, each in themselves tiny magnets. When a 
magnetic substance is magnetized, the small molecular magnets point in 
the same direction. When the magnetic substance is not a magnet, the 
small molecular magnets point in different directions. 

Exp. 22, Part I. By testing a number of common substances, de¬ 
termine which of them are magnetic and also which of them act as mag¬ 
netic screens. 

A. Magnetic substances. Hold a bar magnet close to small bits of 
various substances, such as glass, paper, sand, wood, iron, zinc, lead, 
nickel, tin and common “tin.” Common tin is sheet iron covered with 
tin. 

Also find out whether red hot iron is a magnetic substance. To do 
this, suspend a short piece of iron, or of steel watchspring, on a copper 
wire and hold it in a Bunsen flame until all of the iron is red hot. Then 
bring the bar magnet close to the iron. 

Classify as magnetic substances those which are attracted by the bar 
magnet, and as nonmagnetic substances those which seem not to be at¬ 
tracted. 

B. Magnetic screens. Place a small quantity of iron filings or iron 
tacks on a sheet of cardboard. Hold one pole of the bar magnet against 
the under side of the cardboard and move it back and forth beneath the 
bits of iron. Notice whether there is any evidence of magnetic action 
through the cardboard. 

Now place the iron filings or tacks on a piece of sheet iron or com¬ 
mon “tin” and make the same test. Repeat, using sheets of glass, copper, 
lead, brass, thin wood, etc. 

Classify the substances which you tested according as they do or do 
not act as a magnetic screen to cut off magnetic action. 

1. How do your lists of magnetic and non-magnetic substances 

compare with your lists of substances which do and do not act 

as magnetic screens. 

2. Does air act as a magnetic screen? 

3. What effects do the brass case and the glass of a compass have 

on the reading of the needle? 


Laboratory Manual of Physics 


53 


4. In what kind of case would you inclose a watch to protect 
the mainspring and balance wheel from becoming magnetized? 

5. Will a magnet attract a tin can? Explain. 

Exp- 22, Part II. Make a permanent magnet and use it to study the 
theory of molecular magnets. 

(a) Hold one end of an unmagnetized piece of watchspring or hard¬ 
ened steel knitting needle close to a compass and show that it attracts 
both ends of the compass needle. 

1. What two facts does this show with regard to the substance 
which you have thus tested? 

(b) Magnetize the watchspring or knitting needle by stroking it 
from end to end, always in the same direction, with the N-pole lof a bar 
magnet. Bring the magnet which you have thus made close to the com¬ 
pass and determine which end of it is the N-pole. Mark this end with a 
bit of thread. 

2. What kind of pole do you find at the end of-the needle which 
last leaves the magnet? 

See if you can reverse the poles of your magnet by stroking it in the op¬ 
posite direction with the N-pole of the bar magnet. 

(c) Be sure that you know which end of your magnet is the N-pole. 
Then break it into several pieces, being careful to keep the pieces in their 
original order. Test the polarity of each piece with the compass and 
mark the N-poles. 

3. Does each piece have one pole or two poles? 

4. Show by means of a diagram the locations of the poles on the 
broken pieces when they are arranged in their original order, 
but slightly separated? 

5. What evidence do you find here for the theory of molecular 
magnets ? 

(d) Suspend one of the short pieces of the broken .magnet by 
means of a copper wire and hold it in a Bunsen flame until it is red hot. 
Allow it to cool somewhat and test it with the compass. 

6. Is it still magnetized after heating? Is it it still a magnetic 
substance? 

7. In view of the fact that the velocity of the molecules in the 
iron increases with an increase in temperature, how does this re¬ 
sult lend support to the theory of molecular magnets? 

8- Can you explain by means of this theory why red hot iron is 
not a magnetic substance? 


23. STATIC ELECTRICITY 

Electrostatic effects were known for hundreds of years before “cur¬ 
rent electricity” was studied. With the use of high voltages for the trans¬ 
mission of electrical energy and the development of radio, static effects, 
or the effects of charges as distinguished from currents, are of increas¬ 
ing importance. 

An excess of electrons, such as is produced on sealing wax or ebonite 
when rubbed with flannel, is called a negative charge. A deficiency of 
electrons, such as is produced on a glass rod when rubbed with silk, is 
called a 'positive charge. 


Exp. 23, Part I. Make and use a pith-ball electroscope. 

From a convenient support hang by silk threads two pith balls or bits 
of cork. Charge an ebonite rod by rubbing it with flannel and bring it near 
the pith balls. Note that they are attracted to the rod. Manipulate the rod 

so that it comes in contact with 
various parts of the pith balls. In 
a short time the pith balls will be 
repelled and will repel each other. 
Bring the hand near the pith 
balls. Rub the glass rod with silk 
and observe the action of the balls 
under various conditions. Vary 
the experiment in a number of 
ways (including dampening the 
pith balls), observing the condi¬ 
tions you impose and the results 
you secure. 

1. Why were both balls first 
attracted and then repelled? 

2. If at any time you had two 
unlike charges, describe the 
conditions and results and ex- 

Fig. 25. Attraction and repulsion of electric plain them. 

charges. g If at any time you had 

two like charges, describe the conditions and results and explain them. 



Exp. 23, Part II. Verify the laws of attraction and repulsion. 

Suspend in a stirrup, hung on a silk thread, an ebonite rod which 
has been charged by rubbing it with flannel, Fig. 25. Bring near it a 
glass rod which has been rubbed with silk. Observe the effect on the 
ebonite rod. 

Charge another ebonite rod, or a stick of sealing wax, by rubbing it 
with flannel, hold it near the suspended rod and observe the effect. 


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Laboratory Manual of Physics 


55 


1. What confirmation have you of the law that like charges re¬ 
pel? That unlike charges attract? 

Exp. 23, Part III. Charge a leaf electroscope. 

A. To charge an electroscope by contact. If a charged body like a 
rod of ebonite is brought directly in contact with the knob of the electro¬ 
scope, Fig. 26, there is great danger that leaves will be torn from the 
support. Consequently an instrument called a proof plane is used to 
transfer small charges from the electrified body to the electroscope. A 
proof plane may be made easily by sticking a cent on the end of a glass 
rod with sealing wax. Touch the proof plane to the charged rod and then 
to the electroscope. Repeat until the leaves diverge 
about 45 degrees. The charge on the electroscope will 
have the same sign as the charge on the rod; hence, 
if a glass rod is used, the sign of the charge will be 
positive. 

Bring the charged glass rod near the electroscope. 
What do the leaves do? Explain. 

Bring a charged ebonite rod slowly toward the 
electroscope. Watch the leaves carefully. What do 
they do first? What next? 

B. To charge an electroscope by induction. Bring 
an ebonite rod toward the electroscope until the leaves 
diverge about 45 degrees. Keeping the rod in this po¬ 
sition, touch the knob of the electroscope with the 
hand for an instant. Then remove the rod. What do 
you observe? Is the electroscope charged positively 
or negatively? 

1. On what principle does the leaf electroscope work? 

2. Why is it important to look for the first motion of the leaves 
when determining the sign of an unknown charge?* 

3- When an electroscope is charged by induction is its charge 
like the original charge or opposite in sign? 

Exp. 23, Part IV. Charge two metal balls by induction. 

Suspend two metal balls on silk threads. If you are skillful you can 
hold the threads in the hand. Bring a heavily charged ebonite rod close 
to them, in a line with their centers, and while the rod is in this position 
separate the balls. Now bring each ball in turn near the electroscope 
and determine its charge. Then touch the two balls together and again 
hold them near the electroscope. 

1. When two parts of an insulated body are charged by induc¬ 
tion, which part is charged like the original charge? Explain 
why this should be the case. 

2. How does this experiment answer the question as to whether 
any electricity is produced when a body is charged by induction? 



Fig. 26. Leaf elec- 





24. THE VOLTAIC CELL 


Although the modern development in electricity is largely due to 
the use of electrical generators run by steam or water power, there are 
actually more voltaic cells used to-day than ever before. Nearly every 
automobile carries a battery of three cells. Every flash light has from one 
to three cells. Cells are also used extensively in operating telephones, 
radio sets, door bells, and signalling devices. 


Exp. 24, Part I. Set up a single cell and study its action . 

A. Chemical action. Fill the cup, Fig. 27, with dilute sulfuric acid 
(one part of acid to twenty parts of water), insert a zinc^element and 
observe whether any bubbles of gas are form- 
ed. Repeat with a copper element. 

Clamp both elements in place, put them 
in the solution and observe the bubbles. Join 
the binding posts with a piece of copper wire 
and note what happens at the elements. 

1. Describe the action of the acid upon 
the elements under each of the four con¬ 
ditions mentioned above. 

B. Amalgamation. Repeat the above 
experiment, using an amalgamated zinc ele¬ 
ment. Zinc may be amalgamated by dipping 
it in sulfuric acid and then rubbing a little 
mercury over its surface. 

2. (a) Are bubbles of gas formed when 
amalgamated zinc is placed is placed in 
dilute sulfuric acid? (b) How does 

this result differ from that obtained with unamalgamated zinc? 

(c) What is local action? 

C. Production of current, (a) Connect pieces of copper wire to each 
binding post ef the cell and touch each of these wires in turn to the 
tongue. Also hold both wires to the tongue at the same time. 

3- What is the effect of an electric current on the tongue? 

4. The current flows from the copper to the zinc through the 
tongue. Can you tell, by the tongue, the direction in which a 
current flows? 

(b) Place a galvanoscope 1 so that the turns of wire are parallel to 
the compass needle and connect the cell to the single-turn coil. Place the 



Firr. 27. Daniel, cell. 


1 See Galvanoscope in the appendix. 


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Laboratory Manual of Physics 


57 


needle under this turn, and observe the effect. If the effect is not large, 
use more turns. Reverse the connections and note the effect. The mag¬ 
netic effects of a current will be studied in Exp. 
25; in the meantime we shall use the galvano- 
scope as a means of roughly determining the 
strength of currents. 

D. Polarization. Connect the cell to the 
largest number of turns of the galvanoscope 
and insert sufficient No. 36 German-silver wire 
to bring the deflection down to less than 45 de¬ 
grees. 1 Remove the elements; dry the zinc and 
heat the copper in a Bunsen flame in order to 
free it from hydrogen gas. 

Replace the elements and observe the de¬ 
flection of the galvanoscope needle as soon as 
it comes to rest. Observe the needle and also the action at the plates. 
After a minute or two, short circuit the cell, i. e. touch both binding posts 
with a short piece of copper wire. Observe what happens in the cell, es¬ 
pecially at the surface of the copper plate. While the cell is short cir¬ 
cuited, the galvanoscope will show no readings. Remove the short circuit 
and note the reading of the needle. 

5. Describe the nature and effects of polarization as you have 

observed them. 

Exp. 24, Part II. Set up a Daniel cell and study its action. 

(a) Pour a part of the acid into the porous cup and place the cup 
in the tumbler. The acid should be at about the same height in each. 
Insert the plates in the clamps, with the zinc plate in the porous cup. 
Take observations similar to those of Part I, D. 

1. What effect does the porous cup have on polarization? 

(b) Remove the elements and porous cup from the tumbler and 
replace the acid in the tumbler with copper sulfate solution. Replace 
the elements, the zinc in the acid and the copper in the copper sulfate. 
You now have a Daniel cell. Make observations similar to those under 
(a), above. 

2. Does the copper sulfate of a Daniel cell prevent polarization? 

3. Explain why no bubbles form on the copper element when 

it is surrounded by copper sulfate. 



“If you do not have enough resistance wire to reduce the deflection sufficiently, move 
the compass along the frame away from the coil. The experiment will be more successful, 
however, if resistance wire is used. 



25. MAGNETIC EFFECTS OF AN ELECTRIC CURRENT 


It was only a little over a hundred years ago that Oersted, a Danish 
physicist, made the important discovery that an electric current has a 
magnetic field. This was the beginning of the science of electromagne¬ 
tism. The apparatus with which 
Oersted experimented Consisted of a 
voltaic cell, some wire, and a mag¬ 
netic needle, much like those which 
we will use in this experiment. 

The observations which are 
taken in this experiment should in 
each case agree with the predictions 
Fig. 29. The right hand rule for the direction f rom the right hand rule. This rule 
of the field surrounding a wire carrying current. ^ . f yQu gragp & wire in 

the right hand with the thumb in the direction of the current, the fingers 
represent the direction of the circular magnetic field which surrounds 
the wire, Fig. 29. 



Exp- 25. Study the magnetic effects of an electric current. 


A. Magnetic effects of a single conductor. Connect a cell through 
a reversing switch to a long loop (2 or 3 meters) of copper wire, Fig. 30. 
By -using the same length of copper wire throughout the experiment, 
the resistance of the circuit is kept constant, and the current remains at 
nearly the same value. If an open circuit (polarizing) cell, such as a dry 
cell, is used, it is a good plan to close the switch only when the observa¬ 
tions are being taken. Why? 

(a) Place the wire across the top of the compass parallel to the 
needle, with the current flowing from South to North. Close the switch 
and observe the direction and amount of the deflection. Reverse the cur¬ 
rent and note the deflection. 


1. Show that these results could have been predicted from the 
right hand rule. 



(b) Place the com¬ 
pass near the North edge 
of the table and hold the 
wire in a vertical position 
close to the N-pole of the 
needle, with the current 
going down. Using the 
right hand rule, predict 

What will happen. Close 0 Magnetic effect of a cnrrent. 

the switch and see it you 

were right. Reverse the current and note the eifect. 

(c) Place the wire under the compass with the current flowing 
from North to South. Observe the deflection. Reverse the current and 
again -observe the deflection. 


rr>8] 


















Laboratory Manual of Physics 


59 


2- Show that these results agree with the right hand rule. 

B. Magnetic effect of a single loop of wire carrying a current. 

3. Consider the results of the above experiments and decide how 
the needle will be affected by a current which passes around the 
needle in one complete loop, across the top from South to North, 
down on the North side, back across the bottom and up on the 
South side. 

(a) After answering the above question try it and see if your an¬ 
swer is correct. 

(b) Turn the loop of wire so that it is at right angles to the needle 
and note the effect. Reverse the current and observe. Try other positions 
for the loop. 

4. In what position, relative to a needle, should a loop be placed 
to exert the maximum moment (twist) upon the needle? 

(c) Fold the loop back on itself so that the two parts are side by 
side. Place the loop in various positions with respect to the needle and 
observe the effect. 

5. How do you account for the effects observed in (c) ? 

C. To find the direction of an electric current. Have someone con¬ 
nect two wires to the cell and cover the cell with a piece of cloth. De¬ 
termine which way the current flows from the cell, (a) by touch¬ 
ing the tongue (see Exp. 24), (b) by using the compass and applying 
the right hand rule. After reaching your conclusions remove the cloth 
and see if you were right. 

1. Explain briefly how to determine the direction of an electric 
current if the source of the current is inaccessible. 


Suggested form of record 


Position of wire 

Direction of current 

Direction and amount 
of deflection 

Above needle 

S to N 


N to S 


In front of N 
end of needle 

Down 


Up 


Under needle 

N to S 


S to N 


Loop parallel to needle 

S to N on top 


N to S on top 


Loop at right angles to 
needle 

E to W on top 


W to E on top 



























26. MAGNETIC PROPERTIES OF COILS 


If you have not as yet performed Exp. 25, you should read the intro¬ 
duction to that experiment. In it you were given the right hand rule tor 
finding the direction of the magnetic field surrounding a linear conductor. 
There is a similar rule for the field of a coil carrying a current. It you 
grasp the coil in the right hand, Fig. 31, with the fingers pointing m the 
direction of the current, the thumb points in the direction of the north 
pole of the coil. It is quite unnecessary for you to tax your memory m 
regard to these two rules beyond remembering that you grasp with your 
right hand and that the thumb and fingers represent the directions ot 
either field or current, as the one or the other fits the case. 


Exp. 26, Part I. Make an electromagnet and study its magnetic field. 

A The magnetic field of a helix. Make a helix by wrapping a long 
piece of copper wire around a soft iron rod. Remove the rod and con- 
p nect the wire to the cell 

through a reversing 
switch. Trace the cur¬ 
rent, apply the right hand 
- rule, and make up your 
mind which end of the he¬ 
lix will act as an N-pole. 
Hold a compass near this 
Try the other end. Reverse 





Fi*. 31. The right hand rule for a coil. 


end of the helix and see if you were right, 
the current and observe the effect. 

1. What is the direction of the field inside a helix carrying a 
current? 


B. A study of the electromagnet, (a) Insert the iron rod in the 
helix or wind a new coil on the rod. Connect the coil to the cell through 
a reversing switch. Turn on the current, trace its direction, and by mak¬ 
ing use of the right hand rule, decide which end of the helix is the N-pole. 
Test your conclusion with the aid of the compass. 

Compare the strengths of the fields of the helix with and without 
the iron core. 

2. iShow that the right hand rule enables one to tell which pole 
of an electromagnet is the N-pole? 

3. What is the effect on the strength of the field, or introduc¬ 
ing an, iron core into a coil carrying a current? 

(b) Wind about twice as^iany turns of wire on the iron core and 
note whether the strength of the poles is increased. 

4. If you wind a coil on a rod from one end to the other and then 
back again (like thread on a spool) is the magnetic effect reduced 
by the second layer? Explain fully. 

5. What is the function of the iron core in an electromagnet? 


[60] 











Laboratory Manual of Physics 


61 


(c) See how many tacks the electromagnetic will pick up. Remove 
the core and try the, helix alone. Next wind the horseshoe-shaped core 
with about thirty turns of wire. Test its polarity and strength. Add 
a great many more turns and test the result. 

6. What can you say in regard to the relative lifting forces of 
bar and horseshoe magnets? 


Exp. 26, Part II. Set up and study a d’Arsonval galvanometer. 

In the galvanoscope we have an example of a stationary coil and a 
movable magnet. D’Arsonval designed an excellent galvanometer in 
which he used a powerful fixed magnet and a light movable coil. Nearly 
all high grade direct current voltmeters are of the 
“moving coil” or d’Arsonval type. 

Set up the d’Arsonval galvanometer, Fig. 32, 
level it, and connect a short piece of copper wire 
across its terminals to act as a shunt. Then con¬ 
nect the galvanometer with the source of current 
through a reversing switch. If possible trace the 
wires from the cell through the galvanometer coil 
and decide which end of the coil will be the N- 
pole, which end will be the S-pole, and also which 
way the coil will turn when the switch is closed? 

Are you right? 

Learn how to adjust the zero point of the 
galvanometer. 

7. (a) How do you adjust a d’Arsonval 
galvanometer so that the pointer reads zero 
on the scale, before the current is turned on? 

(b) How do you adjust a galvanoscope so 
that it will read zero at the start? 

8. Why is the pointer of a d’Arsonval gal¬ 
vanometer attached to the moving coil in such 
a way that the coil must be parallel to the 
lines joining the poles of the magnet when Fi s- 32 - 
the pointer reads zero? 

9. Why may a d’Arsonal galvanometer be set up in any position 
relative to the earth’s magnetic field, while a galvanoscope must be 
placed with the coils parallel to the earth’s field? 

10. Which is more sensitive to current, the galvanometer or the 
galvanoscope which you have used? 

11. Explain the advantages which would be gained by (a) a stronger 
magnet, (b) a larger number of turns on the coil? 



D’Arsonval galvan-' 
ometer. 

















27. THE ELECTRIC BELL 


Exp. 27. Study the operation of an electric bell and the arrange¬ 
ment of bell circuits. 

A. Construction and operation of an electric bell. Connect an elec¬ 
tric bell with a cell, through a switch or push button. Trace the current 
from the positive terminal of the cell through the bell and decide on the 
polarity of the poles of the electromagnet. Test the polarity with a com¬ 
pass. In your note book make a drawing of the bell and connections. In¬ 
dicate the direction of the current and the polarity of the magnets. 


1. (a) Are the two poles of opposite polarity? (b) Are the 

two coils wound in the same direction? (c) How can both an¬ 
swers be correct? 


2. What is the purpose of the small spring on the armature? 

3. Explain what makes the hammer move toward the bell and 
what makes it move away from the bell. 



Find a way to connect the bell so as to make 
it a single stroke bell. 

B. Repair of an electric bell or bell circuit. 
The instructor should disable the bell or circuit 
and require the student to tind and repair the 
trouble, so that he may be able to make similar 
repairs at home. 

C. Electric bell circuits, (a) Two bells 
from one push button. Make a drawing of the 
wiring necessary when one push button is used 
to operate two electric bells. Two cells are to be 
used. 

(b) House circuit for two push buttons. 
Two bells, one on the first floor and one on the 
third floor, are to be operated by the front door 
push button, and two buzzers placed beside the 
bells are to be operated by the push button at 
the kitchen door. The entire circuit is to be op¬ 
erated by two cells placed in the basement. Make 
a drawing of the connections. Four groups of 
students should agree on a plan for the wiring 
and by combining their equipment try it out. 




















28. ELECTROMOTIVE FORCE AND INTERNAL RESISTANCE 

OF CELLS 


Two conductors immersed in a liquid which acts on one of them more 
than the other constitute an electrolytic cell The electromotive force, 
or electrical pressure exerted by the cell, depends on the material of which 
the two conductors are made and on the nature of the liquid, or electro¬ 
lyte, as it is called. 

In this experiment a number of different cells will be assembled and 
their electromotive forces compared. 

Exp. 28, Part I. Study the factors 'upon which the electromotive 
force of a cell depends. 

A. Cells having the same electrolyte but different plates. Set up a 
simple cell, using zinc and copper plates and dilute (1 to 20) sulfuric 
acid as the electrolyte. Connect the cell to the galvanoscope 1 , using the 
largest number of turns. Insert sufficient No. 36 German silver wire to 
make the deflection less than 45 degrees. The galvanoscope 1 serves as a 
crude voltmeter. Record the reading of the galvanoscope and note which 
way the needle deflects and to which terminal the copper is connected. 
Copper is positive with respect to zinc and later on when you use other 
metals the plate connected to this same terminal is positive with respect 
to the other plate if the needle deflects the same way; if it deflects the 
opposite way the plate is negative. 

Replace the copper plate with plates of aluminum, lead, carbon and 
any other materials available and record the deflections of the needle. 
Note also which metal is positive with respect to the other. 

Next replace the zinc plate with a lead plate and try it in combina¬ 
tion with all the other plates, taking observations as before. Arrange 
the data neatly as indicated in the table at the end of this experiment. 

1. Which pair of substances gives the greatest electromotive 

force when immersed in sulfuric acid? 

2 Arrange the substances in a series so that each substance is-j- 

with respect to those following it and — with respect to those pre¬ 
ceding it. 

B. Cells having the same plates but different electrolytes. 

Set up a simple cell with zinc and copper plates and first use as 
an electrolyte dilute sulfuric acid. Follow the directions for the use of 
the galvanoscope given in Part I and measure the electromotive force. 
Replace the sulfuric acid, first with dilute nitric acid, and then with di¬ 
lute hydrochloric acid, copper sulfate, and water, in turn. Measure the 
electromotive force of each cell. The plates should be thoroughly washed 
before using them with each new electrolyte. 


J A low range voltmeter should be used if one is available. A voltmeter is connected di¬ 
rectly to the cell. See Voltmeter and Galvanoscope in the appendix. 



64 


Laboratory Manual of Physics 


3. Which electrolyte gives the greatest electromotive force with 
zinc and copper plates? 

Exp. 28, Part II. Study the effect of internal resistance on current. 

The electromotive force of a cell depends only upon the material of 
the plates and the electrolyte 1 . It is in no way affected by the size of 
the plates and the distance between them. The size of the plates and dis¬ 
tance between them is, nevertheless, of the greatest importance in the con¬ 
struction of cells as we shall see in this experiment. 

Set up a simple cell (zinc-copper, sulfuric acid) and connect it to 
the largest number of turns Qf a galvanoseope, using a sufficiently long 
piece of German silver wire in the circuit to bring the deflection down to 
about 40 degrees 2 . Wait until polarization is complete and observe the 
deflection. Then carefully and slowly raise the elements until they are 
clear of the solution and observe the deflection as you do so. Remember 
that, by Ohm’s law, the current in a circuit is equal to the electromotive 
force in the circuit divided by the total resistance and that the electro¬ 
motive force is required to drive the current not only through whatever 
is connected to the cell but also through the cell itself. 

1. Why does raising the elements in the solution increase the 
internal resistance of the cell? 

2. Does the reduction of the current mean that the electromotive 
force is reduced? If not, what does it signify? 

Replace the plates and place them just as near together as possible 
without touching. Read the deflection of the galvanoseope after it be¬ 
comes constant. Then separate the plates as far as possible and again 
note the deflection. 

3. What effect does increased distance between plates have on 
the internal resistance of the cell? 

4. If you wish to construct a cell and get from it a very large 
current what would you attempt to do in respect to (a) size of 
plates, (b) distance between plates? 

5. Automobile storage batteries are made so that they will de¬ 
liver a very large current. Examine such a battery, (a) Why are 
there so many plates? (b) Why are they so close together? (c) 
What then can you say about the internal resistance of such a 
storage battery? 

Exp. 28, Part III. Study the effect of series and of parallel connec¬ 
tion of cells. 

A. Series connection. Connect two cells in series, i. e. with the 
positive terminal of one connected to the negative terminal of the other. 

^Polarization affects the electromotive force of a cell because it changes the material of 
the plate, substituting a surface of hydrogen for the copper. 

2 The smaller the deflections of the needle of the galvanoseope the more nearly are they 
proportional to the current. If a voltmeter is used, its readings are proportional to the current. 



Laboratory Manual of Physics 


65 


Connect the free terminals to a voltmeter and read the combined voltage 
of the cells in series- Also read the voltage of each cell separately. 

If a voltmeter is not available a galvanoscope may be used. If enough 
resistance can be placed in series so that the deflection is less than 25 de¬ 
grees the deflections will be almost proportional to the voltage. 

1. What is the effect on the total voltage when cells are con¬ 
nected in series? 

B. Parallel connection. Repeat the experiment with the cells in 
parallel. 

2. What is the effect on the voltage when cells are connected in 
parallel? 


Suggested form of record 

Part I 

A. Cells having same electrolyte but different plates: 


Pair of plates 

Electromotive force 

Pair of plates 

Electromotive force 

Zinc — copper + 


Lead_aluminum— 


Zinc__ aluminum __ 


Lead carbon— 


Zinc_carbon— 


Lead copper— 


Zinc _lead- 













B. Cells having the same plates but different electrolytes: 


Electrolyte 

Plates 

Electromotive force 

Sulfuric acid 

Zinc ( —) copper ( + ) 


Nitric acid 

(-—) (-) 


Hydrochloric acid 

(-—> <-—) 


Copper sulfate 

(—-) (-—) 


Water 

(—-) (—-> 



Part III 

Voltages of cells in series -- 
Voltages of cells in parallel 


Voltage of first cell- 

Voltage of second cell-.. 














































29. ELECTROLYSIS AND ELECTROPLATING 


When a small amount of sulfuric acid (H 2 S0 4 ) is placed in water, 
the molecules, of their own accord, break up into hydrogen ions and sul¬ 
fate ions. Each hydrogen (H+) ion is an atom of hydrogen with a sin¬ 
gle positive charge and each; sulfate ion (:S0 4 ) consists of one sul¬ 
fur atom and four oxygen atoms with a double negative charge. If 
there is an electric field in the solution, such as is produced when two 
plates, connected to a battery, are placed in the solution, the H+ ions are 
attracted toward the negatively charged plate and the S0 4 ions are at¬ 
tracted toward the positively charged plate. 

When the hydrogen ions reach the — plate they acquire electrons 
from the plate and become uncharged hydrogen gas which rises to the 
surface. 

When the S0 4 ions reach the -f- plate they give up their elec¬ 
trons and at the same time unite with hydrogen from the water. Since 
water is composed of hydrogen and oxygen, oxygen is freed and it rises 
to the surface as bubbles. 

It will be noted that no sulfuric acid is used up in this process but 
that the water is decomposed into its two constituents, hydrogen and oxy¬ 
gen. This process is known as electrolysis and it is used commercially 
to produce large quantities of hydrogen and oxygen. 

An enormous electro-chemical industry has been built upon the in¬ 
dustrial applications of the principles of electrolysis. Electroplating of 
copper, nickel, silver, etc. is one of the best known applications but elec¬ 
trolysis is also used in the smelting of ores and the refining of metals like 
copper. The discovery of an electrolytic method of extracting aluminum 
from its ores was the necessary factor in the development of the alumi¬ 
num industry, for it brought the price of aluminum to the point where 
the metal could compete in the industrial market. 

Exp. 29, Part I. Decompose water by electrolysis. 

Wrap the bare ends of two lengths of copper wire around two nails. 
Connect the wires to a source of electric current and insert the nails in a 
tumbler filled with dilute sulfuric acid (1 to 20). Observe the bubbles 
of gas which appear. 

1. (a) Is the nail from which bubbles come more freely, con¬ 
nected to the + or — terminal of the battery? (b) Is this gas 

hydrogen or oxygen? 

2. Is gas given off at the other nail? If so, what gas? 

3. Give a brief explanation of the reason why the hydrogen and 

oxygen appear at the respective nails. 

Exp. 29, Part II. Electroplate a coin with copper. 

Scrub a nickel (coin) with soap and water and rinse it thor¬ 
oughly. Attach it to a copper wire with a paper clip or some other sim- 


[66] 


Laboratory Manual of Physics 


67 


pie device. Attach another copper wire to a carbon plate or rod and place 
the coin and carbon in a tumbler of copper sulfate solution prepared 
with distilled water or rain water. Connect the wires to two or more dry- 
cells in series, the nickel to the — terminal. Observe what goes on at 
the nickel electrode and at the carbon electrode. If gas is not formed on 
the surface of the carbon rod, bring it very close to the nickel. Let the 
current pass for five minutes while you answer the following questions: 

4. (a) Into what parts do the mjolecules of copper sulfate 
(CuS0‘ 4 ) divide when in solution? (b) What charge does each 
part carry? 

5. To which electrode do the Cu ions travel, the + or — ? What 
becomes of them when they reach this electrode? Is gas given 
off? 

6. To which electrode do the S0' 4 ions travel? What happens 
when they reach this electrode? Is gas given off? 

Replace the carbon with a strip of copper. 

7. Is any gas given off at the surface of the copper? 

8. When the S0 4 ions reach the copper plate what happens that 
could not happen in the case of carbon? 

9. Is the copper plate being used up? 

Take out the nickel, examine it thoroughly and replace it in the 
solution. Reverse the current and let it pass for a little longer time than 
it did before. Remove the nickel and examine it. 

10. What has become of the copper which was on the nickel ? 

11. If two copper plates are placed in a solution of copper sul¬ 
fate and current is passed through the solution for many hours, 
what will happen to the plates? To the solution? 


30. ELECTRIC CURRENTS BY ELECTROMAGNETIC INDUCTION 

Ifi 1831 Faraday discovered that electrical currents can be produced 
by the relative motion of magnets and coils. His experiments showed the 
way by which mechanical energy could be used to produce electric cur¬ 
rents and thus furnish a cheap source of electrical energy. Practically 
all the modern applications of electricity are directly or indirectly de¬ 
pendent upon the experiments of Faraday. These experiments may be 
repeated with simple apparatus and you can verify for yourself the prin¬ 
ciples of electromagnetic induction. 


Exp. 30. Generate electric currents by electromagnetic induction. 

A. To generate currents by the use of a coil and magnet. The cur¬ 
rents will be produced by the relative motion of a coil 1 and magnet. As 
the currents will not be very large a galvanometer should be used, if one 
is available, but a galvanoscope may be used if necessary. 

Connect the two terminals of the galvanometer with a piece of wire 
to act as a shunt. Call the right hand terminal of the instrument the posi¬ 
tive terminal and connect it to the positive terminal of a cell. Complete 
the circuit and note the direction of the deflection. You are now able to 
tell the direction of any current sent through the galvanometer.. 

Remove the shunt, which is used merely to protect the galvanometer 
from excessive current, and connect the 60-turn coil to the galvanometer 
with a sufficient length of wire so that it can be moved about. 

(a) Pass the coil quickly over the N-pole of the bar magnet and 

note the direc- 

? ^ tionand 

amount of the 
deflection, 
move the 
and note 
deflection. 

1. Complete 
the drawing of 
Fig. 34a by 
showing the di¬ 
rection of the 
current in the 
coil and the 
polarity (N or 
S) of the un- 



N 



'A-- ? 


Re¬ 

coil 

the 


Fig. 34. 


(a) (b) 

Producing currents by electromagnetic induction. 


1 Coils of your own construction are very satisfactory. They may be made of No. 18 bell 
wire or No. 24 double cotton covered wire. It is a good plan to wind one coil with twice 
as many turns as the other. The number of turns necessary will depend upon the sensitivity 
of the galvanometer or galvanoscope and the strength of the magnets. Ooils of 30 and 60 
turns will serve for a galvanometer but it may be necessary to use 100 and 200 turns, or even 
more with a galvanoscope. They may be wound around the fingers, leaving a center hole about 
two inches in diameter, and bound in several places with tape or cord. The instructions are 
written on the assumption that you have coils of 30 and 60 turns and a galvanometer. If a 
galvanoscope is used, the connecting wires must be long enough so that the magnets will not 
affect the needle directly. 


[68 J 



























Laboratory Manual of Physics 


69 


der side and upper side of the coil. 

2. How can you tell from the diagram whether it is necessary 
to do work to produce current by induction? 

3. Complete the drawing of Fig. 34b. 

4. Is the rnotion of a coil with respect to a magnet opposed or as¬ 
sisted by the magnetic effects of the induced currents? 

(b) Reverse the magnet and repeat the experiment. 

5. Make diagrams similar to Fig. 34. 

6. Explain how the law of conservation of energy could be used 
to predict the direction of the induced current. 

(c) Use a coil of half the number of turns and note the effect on 
the deflection. 

7. How does the strength of the induced current change when a 
coil of fewer turns is used? 

B. To produce currents by the use of two coils, (a) Connect the 
larger coil to the galvanometer and the smaller coil to a dry cell through 
a reversing switch. Close the switch and place the second coil on top of 
the first in one quick motion. Observe the effect on the galvanometer. 

8. How do the directions of the currents in the two coils com¬ 
pare? How do the polarities of the faces of the coils in contact 
compare ? 

(b) Quickly remove the second coil from on top of the first and 
observe the effect. 

9. How do the directions of the currents in the two coils compare 
in this case? How do the polarities of the two faces of the coils 
which are next to each other now compare ? 

(c) Open the switch and place the coils in contact. Without mov¬ 
ing the coils, make and break the circuit by means of the switch and note 
the effect. Quickly reverse the current and note the effect. 

10. Explain the results obtained when the current is made and 
broken and reversed. 

(d) Insert a soft iron core through both coils and repeat the pro¬ 
cedure used in (c). 

11. Account for the effect of the iron core on the strength of the 
current. 

12. Make as simple a statement as possible, covering all the cases 
which you have examined, regarding the way in which electric 
currents are induced. 


31. MOTORS AND GENERATORS 


Motors and generators are used so universally that everyone has some 
idea of the extent to which modern life depends on them. The fundamen¬ 
tal principles upon which motors and generators operate are those which 
were discovered a hundred years ago by Faraday and Henry, and which 
have been studied in Exp. 30. Electric generators and motors consist es¬ 
sentially of two parts, one of these parts being the field magnet and the 
other the armature. 

In the machine which you will study, the field magnets are station¬ 
ary and the armature rotates. Like many other electrical machines, it 
may be used as either a motor or generator. When the armature is turn¬ 
ed by mechanical means, this machine acts as a generator and produces 
electrical current. When, on the other hand, an electric current is fur¬ 
nished to the armature, the armature will rotate and produce mechanical 
energy. 

In some electrical machines the field magnets consist of permanent 
magnets, as in the generator of a Ford automobile. All large generators, 
however, make use of electromagnets. 


Exp. 31, Part I. Study the direct-current motor. 



A. A motor with a field produced by permanent magnets. Assem¬ 
ble the motor with the permanent magnets, Fig. 36, and connect the arma¬ 
ture to a dry cell 
through a revers¬ 
ing switch. Close 
the switch and ad¬ 
just the brushes 
until the motor 
runs its best. Open 
the switch and 
throughout the ex¬ 
periment keep it 
open, except when 
you are taking ob¬ 
servations. 


Fig. 35. Small motor and generator. 


1. What i s 
the purpose of 


the commutator? 


2. What should be the position of the poles of the armature at 
the moment when the current in the armature reverses? 


3. Explain, using simple diagrams, why the armature turns. 


Make tests which will enable you to answer the following questions: 

4. What is the effect of reversing the connections in a perma¬ 
nent magnet motor? 


[TO] 







Laboratory Manual of Physics 


71 


5. How is the speed of the motor influenced if the field is weak¬ 
ened by swinging the magnets away from the armature? 

6. What is the result when resistance is inserted in the armature 
circuit? 


B. A motor with a field 'produced by an electromagnet. Remove the 
magnets and connect the electromagnet in series, as indicated in Fig. 36, 
so that the current will pass through both the armature and field one after 
the other. Perform such experiments as will enable you*to answer the 
following questions: 

7. When the current furnished a series motor is reversed, what 
is the effect on the direction of rotation? 

8. Describe two ways in which the direction of rotation of a 
series motor may be reversed. 

Rewire the apparatus as a shunt motor, as indicated in Fig. 36, so 
that the current will divide between the armature and field. Answer the 
following questions: 

9. When the current furnished a shunt motor is reversed, what 
is the effect? 

10. How is the direction of rotation of a shunt motor reversed? 



Exp. 31, Part II. Study the direct-current generator. 

A. The magneto. A magneto is an electrical generator in which the 
field is produced by permanent magnets. Many automobiles use mag¬ 
netos for ignition. They are used extensively in simple telephone instru¬ 
ments. 

Set up a magneto, Fig. 36, using the permanent magnets for the 
field. Connect the armature directly to a sensitive galvanometer or gal- 
vanoscope. Spin the armature with the fingers or with a thread wound 
around the armature shaft. 

1. (a) How do you know that the armature becomes a mag¬ 

net? (b) As a pole of the armature approaches a pole of the 
field does it have the same or opposite polarity? Give a complete 
explanation. 




















































72 


Laboratory Manual of Physics 


2. (a) Does this magneto deliver a direct or alternating cur¬ 
rent? (b) Is the current in the armature itself alternating or 
direct? (c) What change could be made in this generator so that 
it would deliver alternating current? 

B. Generator with an electromagnet for the field. This is the usual 
type of generator and it is exactly the same in principle as the magneto, 
the -only difference being that the field is created by an electromagnet in¬ 
stead of permanent magnets. 

Connect the field coil to a dry cell through a reversing switch and be 
sure to have the switch open when you are not taking observations. Spin 
the armature as before and answer the following questions: 

3. (a) What seems to be the advantage of using an electromag¬ 
net for a field? (b) Are large generators made with permanent 

magnets or field coils? 

4. What effect does reversing the direction of rotation have on 
the direction of the current? Explain. 


Special Experiment. To test a lead storage cell. The liquid in a 
lead storage cell consists of sulfuric acid dissolved in pure water. When 
a cell is being used, or is allowed to stand for a long time, part of the acid 
leaves the water and attaches itself chemically to the cell plates. This 
causes the liquid gradually to become less dense during “discharge.” When 
the cell is being recharged, the opposite process takes place. Thus the 
density of the liquid indicates the degree of charge. 

If acid is added to a cell by anyone except an experienced battery 
man, the density test will have no meaning; a cell may read 1.3' under 
these conditions and still be discharged. Distilled water must be added 
to a cell frequently, but a density test should never be made immediately 
afterwards. 

To test a cell, remove the vent plug and draw enough of the liquid 
into a syringe hydrometer to cause the float to rise. Hold the tube in a 
vertical position so that the hydrometer floats freely and read the density 
on the scale at the surface of the liquid. Return the liquid to the same 
cell from which it was taken and replace the vent plug. A Willard storage 
cell at 80° F. will read 1.28-1.30 when fully charged, about 1.22 when 
half discharged and 1.15 or less when discharged- Use the following for¬ 
mula to find what the density of your cell would be at 80° F.: Density at 
80° F. = 5/2 (Room temp, in °F. — 80°) + reading at room temp. 
What is the degree of charge in the cell tested? 

Repeat the test with the other cells of the battery. 


32. REFLECTION FROM A PLANE MIRROR 


When light strikes an ordinary plane mirror, part of it is reflected 
from the front surface of the mirror and part of it, passing through the 
glass, is reflected from the back surface. If, however, the back of the 
mirror is painted black, reflection will occur only at the front surface; 
this will also be true if the mirror is made of a piece of polished metal. 

A ray of light falling on a surface is called the incident ray, and the 
part of it which is reflected, the reflected ray. The angle between an in¬ 
cident ray and the normal to the mirror is called the angle of incidence, 
while the angle between the reflected ray and the normal is called the 
angle of reflection. According to the laws of reflection, these two angles 
lie in the^ same plane and they are always equal. 

When an object is placed in front of a mirror, rays of light pass 
from it to all points on the mirror. The reflected ray from any one of 
these points can be located by sighting along a ruler at this point and at 
the image of the object in the mirror. As long as the object is not moved, 
the image will not move, ho matter from what angle it is viewed. 

Exp. 32, Part I. Find by experiment the relation between the angle 
of incidence and the angle of reflection. 

Draw across the middle of a sheet of notebook paper a straight line 
MM'. Erect a perpendicular OP to this line near its middle point, Fig. 

37. This perpendicular can be 
drawn with the aid of a protrac¬ 
tor, or it can be constructed with 
dividers, using the method given 
in plane geometry. 

Obtain a plane mirror which 
is blackened on one side and at¬ 
tach it to a small block of wood by 
means of rubber bands. 

Stick a pin vertically at the 
point O. Then place the mirror 
and block on the paper, so that the 
front edge of the mirror is exact¬ 
ly on the line MM', the pin being 
against the glass. Place a second 
pin at a point A 10 or 12 cm 
in front of the mirror. Both pins should be as nearly vertical as possible. 

The line AO marks the direction of a certain ray of light passing 
from the pin A to the mirror. This ray is being reflected at 0 by the 
mirror in a certain fixed direction. To find this direction place your eye 
on a level with the paper and in line with 0 and the image I of the pin A, 
and place a pin B somewhere in this line of sight. 

[781 


/\ 

\ 

\ 










74 


LABORATORY MANUAL OF PHYSICS 


Remove the mirror and pins, draw the lines AO and BO, and indi¬ 
cate the directions which the light traveled along these lines by arrow¬ 
heads. 

1. Which is the angle of incidence, AOP or BOP? Which is the 

angle of reflection? 

Measure the two angles with a protractor or else by drawing an arc 
with 0 as center, which cuts the paths of the incident and reflected rays 
at u and w, and measuring the lines uv and wv, Fig. 37. 

Repeat the experiment with the pin A in some other position. 

Label all parts of your drawings and include on them the observed 
data. 

2. How does the angle of incidence compare with the angle of 

reflection in each of the above cases ? 

3. Explain why a blackened mirror was used. 


Exp. 32, Part II. Find by experiment the location and nature of an 
image formed by a plane mirror. 

On a fresh sheet of paper, draw a line MM' across the middle and 
place the front edge of the blackened mirror on this line as in Part I. 

Stick a pin A 10 or 12 cm in 
front of the mirror, Fig. 38. 

To locate the image of 
this pin, lay a ruler on the pa¬ 
per in some position such as 
BC, so that it points directly 
toward the image I in the mir¬ 
ror. Sight along the edge of 
the ruler until / appears to be 
exactly in line with it and 
then draw along this edge the 
line BC. 

Place the ruler in a new 
position, such as B'C', so that 
it again points toward the 
image I, and draw the line 
B'C'. 



Fig. 38. Locating an image in a plane mirror. 


In the same way draw at 
least two more lines which 
point directly toward the image. Then remove the mirror and extend 
these lines until they meet. Their point of intersection is the location of 
the image I. 

In the same way, locate the image /' of a pin placed at A', the head 
of the arrow AA', Fig. 38. Draw the image IV of the arrow. 








Laboratory Manual of Physics 


75 


1. How does the distance of the image from the mirror com¬ 
pare with the distance of the object from the mirror? 

2. How large is the angle between the mirror line MM' and the 
line All Between MM’ and A'1'1 

3. How does the length of the image of the arrow compare with 
the length of the arrow? 

4. Draw a mirror line on a sheet of paper and in front of it 
draw a triangle. Without using a mirror, construct accurately 
the position which the image of this triangle would have in a plane 
mirror placed on the mirror line. How do the image and object 
compare with regard to shape, size and position? 

5. Why is the image of a tree in a pond inverted? 

6. Why do printers often use a plane mirror to read the type which 
they have set? 


Special Experiment. Concave mirrors, (a) Measure the. focal 
length f of a concave spherical mirror by supporting the mirror in the 
sunlight by means of a clamp and obtaining the image of the sun upon 
a narrow strip of cardboard held in front of the mirror. The focal length 
f of the mirror is the distance from the center of the mirror to the point 
where the spot of light is smallest and brightest. Why? 

If the sun is not shining, throw the image of a distant building on the 
thin strip of cardboard. The distance of this image from the mirror will 
be very nearly the focal length. Why? 

(b) Place in a darkened room a small electric lamp or a candle 
flame at a distance D 0 , about three times the focal length from the mir¬ 
ror. Find the position of the image of this source of light by letting it 
fall on the thin strip of cardboard. Measure the distance of the object 
from the center of the mirror D 0 and the distance of the image Z>,. Com¬ 
pute the focal length / of the mirror from the formula - - -f - - = - 

r>o Lf\ j 

How does this value of the focal length compare with that obtained 
by using the sun or a distant building as the object? 


33. REFRACTION OF LIGHT BY A PRISM 

When a ray of light passes in a slanting direction from air to glass, 
or the reverse, it is bent at the surface separating the two substances. 
This bending, which is called refraction, is a result of the change in the 
velocity of light which takes place when it enters or leaves the glass. 

It is because of refraction that objects viewed through a prism appear 
to have changed their position; the light rays are bent in passing through 
the prism and an observer sees the object in the direction of the rays which 
finally enter the eye. 

The index of refraction of a substance is the ratio of the speed of 
light in air to its speed in the substance. This ratio can be found for 
many substances with the use of very simple apparatus. 


Exp. 33, Part I. Trace the path of a ray of light through a triangu¬ 
lar prism. 

Lay a triangular glass prism in the middle of a sheet of paper, and 
carefully trace its outline with a sharp pencil, Fig. 39. 
Stick two pins in the paper at A and B, placing them 
several centimeters apart and in such a position that 
the line AB makes an angle of about 45 degrees with 
the side of the prism. 

Place the eye on a level with the table and sight 
through the other face of the prism at these pins. 
Stick two pins C and D in such a position that the four 
pins seem to be in the same straight line. 

Remove the prism and pins. Draw the lines AB 
and CD, which represent the paths of the incident and 
emerging rays, respectively. Also draw the line rep¬ 
resenting the path of the ray through the glass and 
indicate the direction of the light along ABCD by ar¬ 
rowheads. 

Using dotted lines, make the following construc¬ 
tions on your diagram: (1) the normal (perpendic¬ 

ular) to the side of the prism at the point where the 
incident ray strikes the glass; (2) the normal to the 
other side at the point where the ray emerges from 
the glass; (3) the path the incident ray would have 
taken if the prism were not there. 

Label all parts of the diagram, including the angles of incidence and 
refraction at the two faces of the prism. 

1. How is a ray of light bent with regard to the normal when it 
enters an optically denser substance? When it emerges into a 
less dense substance? 



Fig. 39. Path of a ray of 
light through a prism. 


[76] 




Laboratory Manual of Physics 


77 


2. Why did all the pins appear to be in the same straight line 
DC I, Fig. 39? 

3. Is a ray passing through a triangular prism refracted toward 
the thicker or the thinner part? 


Exp. 33, Part II. Determine the index of refraction of the glass in 
your prism. 

With a sharp pencil, make a dot A on a sheet of paper. Place a tri¬ 
angular glass prism on the paper with one edge accurately at A, Fig. 40. 
Hold the prism firmly against the paper and carefully trace its outline. 

Lay a ruler upon the paper at BC and, with one eye closed, sight along 
the edge of the ruler at the upright edge A of the prism as seen in the 
glass. The ruler should be placed so as to make a large angle with the 
perpendicular. With a sharp pencil, draw a 
fine line BC along the edge of the ruler to mark 
the line of sight. 

Now place the ruler in a new position 
B'C', again sight along the ruler at the same 
edge A of the prism and draw a second line 
B'C'. 

Remove the prism and extend the lines 
BC and B'C' until they meet at some point I. 

The point I is the position of the image of the 
edge A. 

It is shown in textbooks of physics that 
the index of refraction of the glass can be 
found by dividing BA by BI, Fig. 40. Make 
careful measurements of these distances, es¬ 
timating to 0.01 cm, and calculate the index of 
refraction of the glass. 

Repeat the experiment and compute the 
per cent of difference between the two values obtained. If this exceeds 3 
per cent, more trials certainly should be made. Take the average of your 
values as the index of refraction of the glass. 



Fig. 40. A way of measuring the 
index of refraction of glass. 


1. What is the velocity of light in the kind of glass contained in 
your prism? 

2. Explain how the above method could be used for finding the 
index of refraction of water. 

3. State in your own words why the above method enabled you 
to obtain the index of refraction of the glass. Consult a textbook 
if necessary. 



34. THE RUMFORD PHOTOMETER 


The candle power of a source of light is measured by comparing it 
with a standard candle or with some other light of known candle power. 
This comparison is usually made by placing the two lights near a screen 
and adjusting their distances until each light produces on the screen tne 
same brightness as does the other. The candle power of the light being 
tested can then be computed, using the principle that the candle powers 
of the two sources af light are directly proportional to the squares of their 
distances from the screen. 

If the two lights which are being compared differ much in color, it 
will be found difficult to compare the brightness effects on the screen and 
to determine just when the effects due to the two lights are equal. For 
this reason it is best to compare lights of the same type. If, for example, 
an incandescent lamp is being tested, it can be compared with a similar 
incandescent lamp which has been standardized, say, by the Bureau of 
Standards. 

A standard candle gives 1 c. p. and an ordinary paraffin candle about 
1.25 c. p. A 16 c. p. incandescent lamp gives approximately sixteen times 
as much light as a standard candle. (See table at end of Exp. Add. 8). 

The apparatus used in measuring candle power is called a photometer 
(photos = light). There are many different forms of this instrument, 
but one of the simplest to construct is the Rumford shadow photometer . It 
serves to explain the principle of the more accurate and elaborate instru¬ 
ments and it has the advantage that it does not have to be operated in a 
light-tight box or in a totally darkened room. 

Exp. 34. Construct a Rumford photometer and use it to test the re¬ 
lation between the candle powers of two lights and their distances from 
the photometer screen. 

(a) In a partially darkened room set up a vertical screen AB con¬ 
sisting of a sheet of unglazed white 
paper and a few centimeters in front 
of it place an upright rod R, Fig. 41. 
Stand a single candle L about 15 cm 
from the screen and observe the na¬ 
ture of the shadow cast on the screen 
by the rod R. 

Place a similar candle at L', in 
such a position that another shadow 
is cast on the screen by R. It is a 
good plan to separate the two sources 
of light by means of a dividing screen 
ss'. The two shadows should be made 
nearly to touch each other without 


J For a similar experiment with the Bunsen photometer, see Exp. Add. 10. 

[78] 


A 



Fig. 41. The Rumford photometer. 






Laboratory Manual of Physics 


79 


overlapping. Note that the candle L illuminates the shadow cast by the 
other candle L' and that L' illuminates the shadow cast by L. 

If necessary, trim the candle wicks to make them burn equally and 
then move the candle at L' back and forth until the two shadows are equal¬ 
ly bright. Measure and record the distance from each candle to the 
shadow which it illuminates, not the one which it makes. If the shadows 
are close together, these distances will be mL and mL'. 

(b) Place two candles at L', one directly behind the other and as 
close together as possible. Make sure that all the candles are burning 
properly and then move the two candles at L' back and forth until the 
shadows are again equally bright. Again measure the distances of L and 
L' from m. 

(c) Repeat with three, and finally four candles placed at L'. 

(d) For each of the above four cases, compute the ratio of the 
candle powers of the two sources L and L' and also the ratio of the 
squares of their respective distances from m. 

1. State in your own words and also in symbols the principle 
revealed by a comparison of these ratios. 

2. Why should the distance be measured in each case to the 
nearer of the two shadows? 

3. If a light is moved three times as far away from an object, 
how much brighter must the light be made to illuminate the ob¬ 
ject to the same degree as before? 

4. Explain how your photometer can be used to measure the 
candle power of an incandescent lamp. 

Suggested form of record 


Distance of L from nearer shadow, mL-cm. 

Number of candles at L, 1. 


Candles at L’ 

Distance of L' from 
nearer shadow, mL' 

C. P. at L 

C. P. at L' 

(Distance mL ) a 
(Distance mL') 2 ■ 

1 




2 




3 




4 




























35. CONVERGING LENSES 

A lens that is thicker at the middle than at the edges tends t o con¬ 
verge the rays of light passing through it, while the opposite is true if the 
lens is thinner at the middle. Examples of converging lenses are burning 
glasses, reading glasses and camera lenses. 

When parallel rays of light pass through a converging lens, they are 
refracted to a point called the principal focus. Rays from the far dis¬ 
tant sun and even from objects a few hundred meters away are so nearly 
parallel that they converge to points very close to the principal focus. 

The focal length of a lens is the distance from the center of the 
lens to the principal focus. Its value depends upon the index of refrac¬ 
tion of the lens material and upon the shape of the lens. The thicker 
a lens, in proportion to its diameter, the shorter is the focal length. 

The images formed by lenses and mirrors are of two kinds, real and 
virtual. An image is real when the rays which produce it actually pass 
through the image, making it possible to cast the image on a screen. An 
image is virtual when the rays which produce it only appear to pass 
through the image; a virtual image cannot be cast on a screen, but must 
be observed by the eye. A converging lens produces either real or virtual 
images while a diverging lens produces only virtual images. 

The position of an image, and the matter of whether the image will be 
real or virtual, erect or diminished and enlarged or reduced, depend upon 
both the distance of the object from the lens and the focal length. 

Exp. 35, Part I. Measure the focal length of a converging lens. 

(a) Set up a reading glass in the sunlight, with the lens turned di¬ 
rectly toward the sun. Place a white screen back of the lens and adjust 
its distance until the image of the sun on the screen is as nearly as possible 
a point. Measure the distance from the center of the lens to the image. 
Make two more trials and take the average as the focal length / of the lens. 

If the day is cloudy, obtain the focal length by pointing the lens 
through an open window so as to obtain on the screen a sharp image of 
a house, a brick wall or some other well-lighted object, a hundred or more 
meters distant. 

1. Why is the distance from the image of the sun to the lens the 

focal length? 

(b) Measure the distance from the center of the lens to the screen 
when a sharp image is obtained of an object about 25 meters away. 

2. How does the value obtained in (b) compare with the focal 

length? 

3. Explain why it is not necessary to focus a small box camera. 

Exp. 35, Part II. Study the images formed by a converging lens. 

Arrange the apparatus as in Fig. 42, using the same lens as in Part 

■ [ 80 ] 


Laboratory Manual of Physics 


81 


I. The object is the screened opening at 0, the lamp behind this opening 
serving merely to increase illumination. 



Fig. 42. Method of studying the images.formed by a converging lens. 


Case 1. Place the lens so that its distance from the object 0 is more 
than 2 /. If, for example, f was found in Part I to be 17.2 cm, place" the 
lens more than 2 X 17.2 cm from the object. 

Now move the screen I back and forth until the image formed upon 
it is as sharp as it is possible to get it. Then measure the distance D 0 from 
the center of the lens to the object O, and the distance D { from the center 
of the lens to the image I. 

Also measure the width L x of the image and the width L 0 of the ob¬ 
ject. 

Record your observations in a table similar to the one given at the 
end of this experiment. Under “Remarks,” state whether the image is 
(a) magnified or reduced, (b) real or virtual, and (c) erect or inverted. 
To find whether the image is erect or inverted, hold a pencil near one cor¬ 
ner of the object and observe its position on the image. 

Calculate , £>“ and / ’ ex P ress i n g fractions decimally. 


Cases 2 and 3. Place the lens in the positions given in the first column 
of the table and, for each position, repeat the above procedure. 

Case 4. With D 0 = /, find whether it is possible to cast a distinct 
image on the screen. 

Case 5. With D 0 < /, try to find a position for the screen such that 

a sharp image will be formed. If unsuccessful, look through the lens 
toward the object and describe what you see. 

1. How does the position and nature of the image change as an 

object is brought nearer to a convex lens? 














82 


Laboratory Manual of Physics 


2 . 


How do the values for 




compare with the value 


for 

3. 


1 obtained in Part I? State the formula thus verified. 
What information does the ratio Li/L 0 give you? 


4. How does D\/D 0 compare in each case with Li/L 0 ? What, 
then, is the relation between the relative sizes of image and object 
and their relative distances from the lens? 

5. When you read fine print with a reading glass, do you make 
use of a real image or of a virtual image? 


Suggested form of record 

Part I 


Lens No_ 

(a) Focal length /: 

Trial 1_c-m ; trial 2_cm ; trial 3-—cm ; 

Average-cm. 

(b) Approximate distance of object_m; distance from lens to image-cm. 


Part II 

No. of lens- 

Focal length, /-cm (From Part I) ; 1/f. 


Width of object, L q —----_cm. 


Position of object 
relative to lens 

D o 

cm 

D i 

cm 

1 1 
— + — 
^0 

4 

cm 


A 

Remarks 

More than 2 f 








Exactly 2 / 








Between 2 f and 1 f 








Exactly 1 / 


Observations: 



i 

Less than 1 f 

1 Observations: 













































36. THE MAGNIFYING POWER OF A SIMPLE MICROSCOPE 


The naked eye sees an object most distinctly when it is about 25 cm 
away. If the object is viewed through a microscope, this distance be¬ 
comes much shorter. 

The simple microscope consists of a single converging lens of short 
focal length /. If the eye is held close to such a lens, and an object is 
placed slightly nearer to the lens than its focal length, one sees a virtual, 
erect and magnified image. 

Linen testers, reading glasses and pocket magnifiers are simple mi¬ 
croscopes. The linen tester consists of a single convex lens and a metal 
screen containing a square hole, both mounted on a frame in such a way 
that the hole is slightly nearer the lens than its principal focus. 

Exp. 36. Measure the magnifying power of a simple microscope. 

Place a meter stick on the table and support the lens just 25 cm above 
the scale, Fig. 43. 

Close one eye and with the other eye look down through the lens at 

the virtual image of the square hole. Notice 
that the image is erect and enlarged. The 
distance Lb from this image to the lens is 
25 cm, this being the distance of most dis¬ 
tinct vision for the normal eye. 

To measure the width Li of the image, 
place the right eye as close as possible to the 
lens. Look through the lens with the right 
eye and at the same time look directly at the 
meter stick with the left eye. Count the 
number of millimeters which the image of 
the hole as seen through the lens by the 
right eye seems to cover on the meter stick 
as seen by the unaided left eye. The num¬ 
ber of millimeters counted is the width Li 
of the image. 

Compute the magnifying power of the 
lens by dividing the width Li of the image 
by the width L 0 of the object. The width 
of the object is found by measuring in mil¬ 
limeters the actual width of the square hole. 

Fig. 43. Method of measuring the Again compute the magnifying power 
magnifying power of a simple 0 f the lens, this time by dividing the image 
microscope. distance D i by the object distance D a . The 

object distance D 0 is found by measuring carefully the distance from the 
square hole to the middle of the lens. 

1. By actual trial, find how far from your eyes you must hold a 



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Laboratory Manual of Physics 


printed page in order to see it most distinctly. How does this dis¬ 
tance compare with the 25 cm taken as the standard? 

2. Show that the magnifying power of a simple microscope is 
approximately 25/f. 

3. When an object is held very close to the naked eye, is the 
image in the eye behind or in front of the retina? 

Suggested form of record 


Lens No- 

Width of image, L.- 

Magnifying power, L ; 

Image distance, D.-2 

Magnifying power, D. 


_mm; width of object, L q - 

!S y| j. — _diameters. 

5_cm ; object distance, D Q —-—.- 

j) -J __diameters. 


.mm. 


cm. 


Special Experiment. The compound microscope. The compound 
microscope in its simplest form is a telescope used for examining objects 
near at hand. 

To make a model compound microscope, use the arrangement shown 
in Fig. 44, and follow the directions given in Exp. 37 A, with the exception 
that the object is to be a candle flame or other illuminated object placed 
near enough to the objective lens to cast an enlarged image on the screen S. 

Measure the distance D 0 from the object to the objective, and the 
distance Zb from the image at S to the objective; then compute the mag¬ 
nifying power of the objective alone by means of the relation Zb/Z> 0 . Also 
compute the magnifying power of the eyepiece, which is 25//. The mag¬ 
nifying power of your compound microscope will then be the product of 
the magnifying powers of the objective and eyepiece. 








37. THE ASTRONOMICAL TELESCOPE 


The astronomical telescope, in its simplest form, consists of a large 
lens of considerable focal length, called the objective lens, and a smaller 
lens of shorter focal length, called the eyepiece. 

The objective lens forms a real, inverted, and diminished image of 
a distant object. This image is viewed through the eyepiece, which merely 
acts as a magnifying glass or simple microscope. The final image seen 
through the eyepiece is virtual and inverted. 

When this telescope is used for astronomical purposes, the inverted 
image causes no inconvenience. If this telescope is to be used to view 
objects on the earth, the image is turned right side up by placing lenses 
or prisms between the objective and eyepiece. 

Exp. 37. Construct an astronomical telescope and measure its mag¬ 
nifying power. 

A. To make a telescope. Arrange the apparatus as shown in Fig, 


44, with the objective lens 
facing an open window. Use 
a reading glass for the ob¬ 
jective lens and a linen test¬ 
er or other short-focus lens 
for the eyepiece. 



Place the eyepiece near 
the edge of the block, and 
with the eye held close to the 
eyepiece lens, adjust the 
white screen S until you can 
see the magnified image of 
its surface distinctly. 


Now move the block sup¬ 
porting the eyepiece and 
screen until a sharp image of 
a distant object is cast on the 
front of the screen by the ob¬ 
jective lens. 


Fig. 44. Model of an astronomical telescope. 


Remove the screen and readjust the eyepiece, if necessary, so that 
when you look through the eyepiece you can see clearly the inverted image 
of the distant object. You are now using the eyepiece as a simple micro¬ 
scope to view the real image which exists at S. 

Measure the distance between the two lenses and compare this dis¬ 
tance with the sum of the focal lengths of the lenses. The focal length F 
of the objective, and the focal length / of the eyepiece, can be found, as 
in previous experiments, by casting on a screen the image of the sun or 
of another distant object. 


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Laboratory Manual of Physics 


1 Explain why this simple relation exists between F + / an( * 
the distance between the lenses. 

2. How would these quantities compare if the telescope were focused 
on an object only a few meters away? 

3. What defects did you observe in the image formed by your tele¬ 
scope? 

B. Magnifying powev. Draw on the blackboard two heavy hori¬ 
zontal lines, about 15 cm apart, and view them through the telescope from 
the other side of the room. Adjust the position of the eyepiece until you 
obtain a distinct image of the lines. 

Now look through the eyepiece with one eye and at the same time look 
at the blackboard directly with the other eye. Direct another student 
where to draw two horizontal lines on the blackboard which seem to have 
the same positions as do the images of the lines seen through the telescope. 

Find the magnifying power of your telescope by dividing the distance 
Li between the two image lines by the distance L 0 between the two object 
lines. Repeat the experiment several times and take the average as the 
best value of the magnifying power. 

Again compute the magnifying power, this time by dividing the focal 
length F of the objective by the focal length / of the eyepiece. 


Suggested form of record 

Objective lens: lens No- 

Eyepiece lens : lens No-*- 

Distance between lenses--cm. 

F + f =_cm. 


; focal length, F-—cm. 

_ ; focal length, f___cm. 


Magnification 


Trial 

Image width 
in cm 

Object width 
in cm 

L o 

Magnification in 
diameters 

L l+ L o 


1 












Observed magnification L { /L o , average value-diameters. 

Magnification, by formula F/f,-diameters. 
























ADD. 1. PRESSURE IN A LIQUID 


If a pencil is plunged endwise into water and then released, it springs 
back into the air. If a strong steel buoy is accidently sunk to the bot¬ 
tom of the sea by having too heavy a weight attached to it, it is often 
found that, when recovered, it has been crushed out of shape, as though 
made of paper. It is evident that water must exert pressure on any im¬ 
mersed surface, and that at considerable depths, the pressure must be 
very great. 

We must distinguish between the terms 'pressure and force as these 
terms are used in physics. Pressure is force per unit area. It is meas¬ 
ured in grams of force per square centimeter, or in pounds of force per 
square inch. 


1. Suppose that six 1 cm cubes of iron are placed on the table, one 
above the other in a vertical column. Find the pressure on the 
table in grams per square centimeter, and also the force in grams. 
Density of iron is 7.4 g/cm 3 . 

2. What would be the pressure and also the force on the table if 
two such columns of iron cubes were placed side by side? 

The open-tube manometer, containing mercury, is 
well adapted for the investigation of the pressure in a 
liquid. This instrument is fully described under Mano¬ 
meters in the appendix. It is lowered into the liquid un¬ 
til the short arm is in the region of the liquid to be in¬ 
vestigated, the end of the long arm being left exposed 
to the air. The difference in the mercury levels in the 
manometer, measured in centimeters, is the pressure in 
the liquid in centimeters of mercury, at the point where 
the liquid touches the mercury in the short arm. 


Exp. Add. 1. Measure the pressure at various depths 
in water and then in gasoline, and find how the pressure 
varies with depth and with the density of the liquid. 

A. To measure the pressure in a liquid with a 
manometer. Place the water, gasoline or other liquid 
whose pressure is to be investigated in a deep vessel. 
Fasten an open-arm manometer tube to a meter stick by 
means of rubber bands, and add mercury to the tube un¬ 
til it stands to a height of about 12 cm in either arm. 
Lower the manometer and attached scale vertically into 
the liouid in the deep vessel, immersing it as far as pos¬ 
sible without submerging the top of the meter stick or the 
top of the long arm of the manometer. Suspend the mano¬ 
meter in this position by means of a pencil laid across the 


Fig. 45. This 
mercury manomet¬ 
er is being used to 
measure the pres¬ 
sure at m in the li¬ 
quid. 


top of the vessel and fastened to the meter stick with a rubber band, 
Fig. 45. 


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Laboratory Manual of Physics 


Surface — 


Observe and record the reading on the meter stick at the surface S 
of the liquid. If this liquid is water or gasoline the surface S will be 
curved upward where it touches the meter stick and sides of the vessel, 
Fig. 46. The lowest point in this curved surface 
is to be regarded as its true position. 

Also read to tenths of a millimeter the posi¬ 
tions, of the mercury at M and m, being sure to 
keep your eye on a level with the mercury and 
_reading the top of the surface (Fig. 59, appen¬ 
dix) . 

Note that S—m is the distance h below the 
surface at which the pressure test is being made, 
while M—m is the height of the mercury column 
supported by the pressure of the liquid. Hence 
M—m is the pressure p, in centimeters of mer¬ 
cury, at a depth h in the liquid. Compute the 
pressure p and depth h. 

Raise the manometer about 15 cm, make the 
necessary readings and compute the new pressure and depth. Continue 
in this way until the liquid has been explored to within about 15 cm of 
the surface. 

Repeat the experiment, using another liquid such as gasoline. 

B. Effect of depth and density of liquid on pressure. In the case 
of both the water and gasoline, compute the quantity depth/pressure, or 
h/p, for each depth h. 

3. Making allowance for experimental error, how do the quo¬ 
tients hi/p lf h 2 /p 2 , h 3 /Ps, etc. for water compare with each other? 

How do they compare with each other in the case of gasoline? 

4. If, in a given liquid, the depth h is doubled, how will the pres¬ 
sure p change? What if h is tripled? 



Water 


Fig. 40. Concave surfaces 
are read at the bottom. 


When a change in one quantity is accompanied by a change in some 
other quantity, the one quantity is said to vary with the other. When 
two quantities change in such a way that their ratio, or quotient, is a 
constant, one of these quantities is said to vary directly as the other. 

5. Is the pressure in a liquid directly or inversely proportional 
to the depth? 

To plot the pressure-depth curve, follow the general directions given 
under Graphs in the appendix. Make the lower left-hand corner of the 
squared paper the origin, plotting depths h along the X-axis and pressures 
p along the Y-axis. After you have plotted the points for water, you will 
find that a straight line can be drawn from the origin so that about half 
the points will be close to one side of the line and the other close to the 
other side. 


6. Use your data to plot a curve showing the relation between 
the pressure in water and its depth. 










Laboratory Manual of Physics 


89 


7. How do we know that this line will pass through the origin? 
Why will it never pass exactly through all of the points when these 
points represent experimental data? 

8. How are two quantities related if the curve drawn to show 
how one varies with the other turns out to be a straight line? 

9. On the same sheet of paper, and using the same scale as for 
water, plot the curve for gasoline. 

10. Read from your curves the pressure of water and of gasoline 
at a depth of 38 cm. Compare the pressures in the two liquids at 
various other depths. Which has the greater pressure at a given 
depth, the denser or the lighter liquid? 

11. Which of the two curves has the steeper slope and why? 
Would the curve for mercury be steeper or less steep than that 
for water? Explain. 

12. In answering Question 1, what assumption did you make with 
regard to the effect of pressure on the density of iron? Is this 
true for a liquid? For a gas? 

Suggested form of record 

Pressure beneath the free surface of_ 


Surface 

reading 

8 

Position of 
mercury in 
short arm 

m 

Position of 
mercury in 
long arm 

M 

Depth h 
in liquid 
in cm 

8-m 

Pressure p 
in cm of 

mercury 

M-m 

h 

"IT 





1 
































(Make a similar table for the second liquid.) 




































ADD. 2. LUiNG-PRESSURE; PRESSURE IN THE CITY GAS MAINS 

Lung pressure and gas pressure are measured with a pressure gage. 
This instrument is also used to find the steam pressure in factory, loco¬ 
motive and heating boilers, and in the pressure cookers used m high 
altitudes by housewives. The barometer is one kind of pressure gage, it 
being used to measure atmospheric pressure and to forecast the weather. 

A familiar type of gage is the one used to find the air pressure in 
an automobile tire. The life of a tire is prolonged by keeping it under 
the proper pressure. This pressure depends upon the size and kind of 
tire. 

In the above cases, pressures greater than atmospheric pressure are 
considered. Pressures less than that of the air can also be measured with 
certain gages. Such pressures are often made use of, as in pumps and 
vacuum sweepers. 

Accurate measurements of fluid pressure can be made with a form 
■of gage known as the manometer. For measuring gas pressure and lung- 
pressure, the open-tube manometer is often used. Its form and operation 
are fully described in the appendix. 

1. Does the open-tube manometer measure the excess of pressure 
above that of the atmosphere, called the gage pressure, or does it 
measure the total, or absolute, pressure? 

2. If the opening of a gas cock has a cross-sectional area of 0.6 
cm 2 and the gas pressure is 24 g/cm 2 , what would be the force in 
grams exerted by the gas on your finger if you held it against the 
opening? 

3. A man blowing steadily into an open-tube mercury manome¬ 
ter produces a difference in levels h of 9.82 cm. What is his ef¬ 
fective lung pressure in grams per cm 2 ? Density of mercury is 
13.6 g/cm 3 . 

Exp. Add. 2, Part I. Measure your lung-pressure and also the re¬ 
duction in pressure which you can produce by the mouth. 

(a) Fill an open-tube manometer half full of mercury and support 
it in a vertical position with a meter stick between the two arms. 

Attach an 8 cm glass tube, rounded on the ends in a Bunsen flame, to 
the short arm of the manometer by means of a piece of rubber tubing. 
If several students use this mouthpiece, sterilize it each time in boiling 
water. 

Blow steadily into the tube for two or three seconds. A quick hard 
blow into the mouthpiece will not give you a true measure of your 
lung-pressure. Why? 

While blowing, pinch the rubber tube and read to tenths of a milli¬ 
meter the positions M and m of the two mercury surfaces. (See Fig. 59 
in the appendix.) Place your eye on a level with the mercury and read 
the top of the mercury surface. 


[ 90 ] 


Laboratory Manual of Physics 


91 


Tabulate data and compute the average value, in centimeters of mer¬ 
cury, of the pressure above that of the atmosphere which you can pro¬ 
duce by blowing. 

Reduce this value to grams per square centimeter. 

4. If you had used water in the manometer, instead of mercury, 
what would the difference in water levels have been? 

5. Would more or less accuracy have resulted from using a long¬ 
er manometer containing water? Explain. 

6. Compute the average lung pressure of the class and find what 
per cent your lung-pressure is of the class average. 

7. The mercury barometer is a form of manometer. Examine 
the one in the laboratory and explain how it measures atmospheric 
pressure. If necessary, refer to a textbook. 

(b) The saipe manometer can be used to measure the reduction in 
pressure which you can produce with the mouth. It may be necessary to 
remove some of the mercury, as great care should be taken not to get 
any of it into the mouth. Apply the mouth to the tube and draw the air 
out as completely as possible. Then follow the same procedure as in the 
case of measuring pressure due to blowing. Record data and find the 
average value of the reduction in pressure in centimeters of mercury. 
Reduce this value to grams per square centimeter. 

Exp. Add. 2, Part II. Measure the pressure in the city gas mains. 

The pressure in the gas mains must be greater than that of the at¬ 
mosphere or the gas will not flow out. If the gas pressure is too low, a 
gas stove often will not operate satisfactorily. 

(a) Fill an open-tube manometer a little more than half full of 
water and support it in a vertical position with a meter stick between 
the two arms. (See Fig. 59 in the appendix.) 

Attach the short arm of the manometer to the gas outlet by means of 
rubber tubing and turn on the gas slowly. Read to tenths of a milli¬ 
meter the positions of the two water surfaces, placing your eye on a level 
with the water and reading the bottom of the water surface (see Fig. 46). 
From these two readings compute the difference in water levels. 

Turn off the gas and repeat this operation several times. This is 
especially necessary if there is much fluctuation in pressure. 

Tabulate data and compute the average value of the gage gas pres¬ 
sure in centimeters of water and also in grams per square centimeter. 

4. What would the difference in the levels have been if you had 
used mercury in the manometer instead of water? 

5. Would more or less accuracy have resulted from using mer¬ 
cury? Gasoline? Explain. 

6. Compute the absolute or total pressure of the gas in grams per 
square centimeter by adding the gage pressure to the atmospheric 
pressure. To do this, read the barometer in centimeters of mer- 


92 


Laboratory Manual of Physics 


cury to the same degree of accuracy that you read the manometer 
(to tenths of a millimeter) ; reduce this reading to grams per 
square centimeter by multiplying by the density of mercury, and 
add this to the gage pressure expressed in the same units. 

(b) A. second very simple apparatus for measuring gas pressure 
consists of a piece of straight glass tubing about 30 cm long which is at¬ 
tached tq the gas supply with rubber tubing. Lower the end of the glass 
tube into a deep vessel -of water and turn on the gas. 

7. Compare the value of the pressure obtained with this appa¬ 
ratus with that obtained with the manometer. Explain why the 
gas pressure can be obtained by this method. 

8. Does this apparatus measure the gage pressure or the abso¬ 
lute pressure? Explain. 


Special Experiment. How to make a barometer • Use a heavy glass 
tube with one end sealed, about 80 cm long and with about a 4 mm bore. 
Cleanse the bore of the tube with sulfuric ether, allowing the ether to 
evaporate. To fill the tube, stand it in a dish at an angle of forty-five de¬ 
grees and drop clean mercury into it with a medicine dropper. Remove all 
air bubbles by means of a long straight wire on the end of which are tied 
securely some small bits of thread. 

With the tube filled to the brim, place the finger over the open end, 
invert the tube, and place the end of the tube below the surface of mer¬ 
cury contained in a wide-mouth bottle or shallow dish. Remove the finger 
under mercury without allowing any air to enter the tube. 

Support the tube in a vertical position, with the open end resting 
lightly on the dish or on a small piece of wire screen, which will allow free 
passage of the mercury. Place a meter stick beside the tube so that the 
zero mark is just level with the mercury in the dish. To find the atmos¬ 
pheric pressure in centimeters of mercury, measure the height of the 
mercury column, reading to the top of the convex surface in the tube. 
Why the top? 

If possible compare your barometer with a standard mercury barom¬ 
eter; if there is any differenec in the readings, learn how to correct the 
error. 

If desired, fasten the barometer tube with wires to a vertical board 
at the lower end of which is a shelf large enough to hold the bottle or 
dish of mercury. The meter stick must be placed on the board in such a 
manner that the scale can be moved up or down. Why is this necessary? 

Will changes in the temperature affect the reading of a barometer? 
Why is it essential that the barometer scale be accurately vertical? Why 
must the zero mark of the scale always be level with the mercury surface 
in the dish before a reading is taken? How is this adjustment of the 
scale made on a standard mercury barometer? 


ADD. 3. THE INCLINED PLANE 


If a cake of ice is too heavy for the iceman to lift, he can get it into 
the wagon by sliding it up a plank or inclined plane. Work is not saved, 
however, by using the inclined plane, for it requires more work to slide 
the ice up the incline than to lift it into the wagon. 

The inclined plane can be studied by constructing a small model like 
Fig. 47. The total work expended in moving the small car from the bot¬ 
tom to the top of the incline is F X l, that is, the product of the force F 
parallel to the plane and the distance l through which the car moves in 
the direction of this force. 

The useful work ac¬ 
complished is W X -h, the 
product of the load or 
weight of car W and the 
vertical distance h. This 
accomplished work is the 
work which would be 
F done if the car were lift¬ 
ed to a height h against 
the force of gravity W 
without the use of a ma¬ 
chine like the inclined 
plane. 

If the distances l 
Fig. 47. Model of an inclined plane. and h are measured in 

centimeters and the forces F and W in grams, the products F X l and 
W X h will each be in gram-centimeters of work. 

The efficiency of a machine is given by the formula, 

. useful work accomplished W X h 

Efficiency - totaI work expen ded ' = FxT* 

In actual practice this efficiency will always be less than 100 percent 
because of friction. If there were no friction, the useful work accom¬ 
plished with the inclined plane would be equal to the work expended on it. 

Exp. Add. 3. Part I. Construct an inclined plane and find its ef¬ 
ficiency. 

Arrange an apparatus similar to that of Fig. 47, with the board 
making an angle of about 30 degrees with the table. 

Add weights to the weight hanger until the car moves steadily and 
slowly up the plane. If suitable small weights are not available it may 
be necessary to place some shot or other material in the car to get a final 
ad j ustment. 

Measure h and l. Find the load W by weighing the car and its con¬ 
tents. Tabulate your values for W, F, l and h and from this data calcu- 



[93] 







94 


Laboratory Manual of Physics 


late in gram-centimeters the work which would be expended and also the 
work which would be accomplished in pulling the car from the bottom to 
the top of the plane, and then calculate the efficiency. 

Repeat the experiment with the plane at a larger angle with the table. 

1. Which is the greater, the expended work or the accomplished 
work, and why? 

2. Why was it necessary to have the car moving uniformly on 
the plane? 

3. Compare F and W and draw conclusions as to why an inclined 
plane is a useful machine. 

4. Does making the slope steeper increase or decrease the effi¬ 
ciency? Why? What is the disadvantage of making the slope 
steeper ? 

Exp. Add. 3. Part II. Test the principle of work by comparing the 
work expended and accomplished on the inclined plane when the effects 
of friction are made negligible. 

Arrange the apparatus as in Fig. 47, with the board at an angle of 
about 45 degrees with the table. 

Place enough weights on the weight hanger to exactly balance the 
weight of the car. Friction cannot be avoided but if the weights on -the 
hanger are such that on giving a slight push to the car it moves with 
equal readiness down the plane and up the plane, these weights plus the 
hanger will then be the force F which would support the car W on the 
plane if there were no friction. 

If suitable small weights are not available, some shot or other material 
can be added to the car to obtain an exact balance: The load W will 
then be the weight of the car and its contents. 

Read and record W, F. I and h and from these data calculate in 
gram-centimeters the work which would be expended and also the work 
which would be accomplished in pulling the car from the bottom to the 
top of the plane if there were no friction. 

1. How does work expended compare with work accomplished 
when there is no friction? What is the efficiency in this case? 

2. The quotient W/F is called the theoretical mechanical advan¬ 
tage. Find its value for the inclined plane which you constructed 
and state what information it gives you. 

3. Explain why. the effects of friction were largely eliminated 
by the method which you used. 

4. Neglecting friction, if an inclined plane is 5 feet high, how 
long must it be to enable a car weighing 2 tons to be pushed up its 
length by a force of 200 lbs? 


ADD. 4. EFFECT OF PRESSURE ON THE BOILING POINT 


The boiling point of water is 100°C only when the pressure of the 
air or vapor above the water is exactly 760 mm of mercury. If the pres¬ 
sure is less, the boiling point is reduced and account must be taken of this 
fact, as we found in Exp. 12. On the other hand, if the pressure is higher 
the boiling point is raised, this being the principle upon which pressure 
cookers operate. 

Although the amount of change of the boiling point with pressure 
has been determined many times, it is an interesting experiment to per¬ 
form. It gives one an opportunity to compare the results of his own work 
with those secured by skilled scientists using the best of equipment. 

Exp. Add. 4. Measure the effect of pressure on the boiling point of 
water. 


Set up the steam generator as shown in Fig. 48, with the open-tube 
manometer filled with mercury to a height of about 8 cm. The pressure 
is to be controlled by the pinchcock and measured by the manometer. 1 

The pressure is increased by closing the pinchcock and thus pre¬ 
venting the steam from escaping freely. The increase in pressure is 

measured in millimeters of mercury by 



taking the difference in the mercury 
levels in the manometer arms. 

(a) First read the thermome¬ 
ter carefully when the pinchcock is 
wide open. The pressure in the boiler 
will then be that of the atmosphere and 
the mercury will be at the same level 
in both arms cf the manometer. 

(b) Now partly close the pinch¬ 
cock until the manometer shows a dif¬ 
ference in level of 40 to 50 mm. As 
soon as the readings of the thermome¬ 
ter and manometer become steady, ob¬ 
serve and record these readings. 

(c) Close the pinchcock still far¬ 
ther, until the difference in the mer¬ 
cury levels is about 20 mm greater 
than before, and repeat the observa¬ 
tions. 

(d) Repeat again, once or twice, 
finishing with a difference in level of 
about 100 mm. 

In each of the above cases, calcu¬ 
late the change produced in the boil¬ 
ing point by a change of 1 mm in the 


Fig. 48. 


Effect of pressure on the boiling 
point. 


’See Manometer in 


the appendix. 


r 95] 



























96 


Laboratory Manual of Physics 


pressure. Take the average of these calculations as the best value for the 
change in boiling point per mm change in pressure. 

1. From your data calculate the change of pressure needed to 
change the boiling point by 1°C. 

2. If the pressure in a pressure cooker is allowed to increase to 
twice that of the atmosphere, what is the temperature of the wa¬ 
ter in the cooker? Of the steam? 

3. Explain why the boiling point is influenced by pressure. 


Suggested form of record 


Temperature of boiling 
point in °C. 





, 

Total increase in tern-' 
perature of boiling 
point, in °C. 






Manometer pressure in 
mm of mercury 






Change in boiling point 
per mm change in 
pressure. 







Average change in boiling point per mm change in pressure_____°C. 

Accepted value- 

Per cent of difference_ 


Special Experiment. Measuring altitude with an aneroid barome¬ 
ter. (a) Measure the vertical distance from the basement to the highest 
accessible point on a building by reading the aneroid barometer at both 
points. Place the barometer face up on the floor and tap the frame gently 
to assist the mechanism in overcoming the friction of the bearings. A 
change in reading of 1 mm means a difference in elevation of approxi¬ 
mately 10.5 meters, providing that the temperatures of the two points 
are the same. 

(b) How much does the elevation of your locality above sea level 
affect the atmospheric pressure? To answer this question, find the ele¬ 
vation of your locality from a map, or other source, and. compute the local 
correction of a barometer. The pressure at sea level is to be taken as 
760 mm of mercury. 































ADD. 5. THE LAWS OF VIBRATING STRINGS 


Exp. Add. 5. Find how the vibration rates of wires are affected 
by length and tension. 

A. Effect of length, (a) Stretch a fine steel wire on the sono¬ 
meter, Fig. 49. Place the movable bridge under the wire at a point about 
two-thirds of the distance from its fixed end and tune the wire until it 
emits a sound of the same pitch as that given by the lowest tuning fork. 

To accomplish this, pluck the wire in the middle with the finger and 
adjust the tension by means of the weights or set screws until the two 
tones are almost in unison. Then complete the tuning by adjusting the 
position of the bridge. When the two tones are almost, but not quite, in 
unison, you will be able to hear beats, which become less and less frequent 
as the two tones approach the same pitch. When the tones have the same 
pitch, the beats will cease. 



Fig. 49. One form of sonometer. 


Measure- the length of the wire. Then displace the bridge and, 
without changing the tension on the wire, repeat the tuning. Again 
measure the length of the wire and compute the per cent of difference 
between this measurement and the first one by means of the formula, 

difference between the two lengths • 


Per cent of difference = 


either length 


X 100. 


If this difference exceeds about 0.5 ! %i, make additional trials, since 
so large an error indicates careless adjustments, changes in the tension 
of the wire or an incorrect determination of the length. Record your 
data in a table similar to the one at the end of this experiment. 


(b) Without changing the tension on the wire, adjust its length 
by means of the movable bridge until it sounds exactly in unison with a 
second tuning fork of higher pitch than the first. Make several trials 
as before and record your results in the table. Repeat the tuning with 
such other forks as are available. 

1. How do the measured lengths of the wire compare with their 
vibration numbers? State the relationship between the rate of 
vibration of a string and its length when the tension is kept 
constant. 


2. Examine the strings on a piano and determine whether long 
or short strings are used for the high notes. 


[97] 














98 


Laboratory Manual of Physics 


3. How much does the pitch of a wire change when its length 
is doubled? To test your conclusion, set the whole sonometer 
wire into vibration by plucking it near one end, and then touch 
the wire exactly at its midpoint with your finger. 

B. Effect of tension. Stretch two fine steel wires of the same di¬ 
ameter on the sonometer. Place the same tension on both wires, using 
enough tension so that the wires produce a low but distinct tone when 
set into vibration. 

(a) Without changing the tension on the wires, make one of them 
just half as long as the other by placing a bridge under its midpoint. 
Call the rate of vibration of the long wire n x and its tension T x . 

4. What is the rate of vibration of the short wire, as compared 
with that of the long wire? How do their pitches compare? 
Without making any further changes in the lengths of the wires, 

adjust the tension on the long wire until it emits a sound of exactly the 
same pitch as that given by the short wire. The vibration rate n 2 of the 
long wire is now 2n±. Read and record the new tension on the long 
wire, calling it T 2 . 

T |2-i 

5. How does the ratio 1 - compare with the ratio-? How 

1 2 W'2 

, ... n x 9 

does ■ . ■ ■ compare with- ! 

sJT 2 n 2 

6. In order to double the vibration rate of a wire, how many 
times must the tension be multiplied? 

(b) Repeat .the experiment with the bridge placed under the short 
wire so that it is only two thirds the length of the long wire, both wires 
being under the same initial tension T x . Again increase the tension on 
the long wire until both wires emit the same tone. Call this final ten¬ 
sion To. 

7. How much was the vibration rate of the long wire changed 
when its tension was increased from T x to T 2 ? 

8. How do —h and compare this time? How do and 

n 2 i 2 

y'rT . 

— compare: 

v t 2 

9. How many times must we multiply the tension on a wire in 
order to triple its vibration rate? 




Laboratory Manual of Physics 


99 


Suggested form of record 

A. Effect of Length 


Length of wire in cm 

Rate of 
vibration 

Trial 1 

Trial 2 

Per cent of 
difference 

Average 

length 











1 






B. Effect of Tension 


ni 

Ti 


Ti 

Vfi 

~ 

grams 

grams 

To 

v rr 












> ’ 
> > > 















































ADD. 6. LAWS OF RESISTANCE USING AMMETER AND 

VOLTMETER 


There are a number of methods for measuring the resistance of a 
conductor. The ammeter and voltmeter method is one of the simplest 
and most convenient. It is based on Ohm’s law, which may be written 


Resistance (ohms) 


potential difference (volts) 
current (amperes) 


It can be seen from the above law that the resistance of a given 
part of a circuit can be found if both the potential drop or voltage over 
this particular part of the circuit and the current in the circuit are 
measured. 


Exp. Add. 6. Measure the resistance of several wires alone and 
token connected in series and parallel. 

A. Measurement of resistance using an ammeter and voltmeter. 
Connect the apparatus as indicated in Fig. 50, using one cell. 1 

Measure the current in each wire and the potential drop over the 
wire. Calculate the resistance of each. 



Fig. 50. Measuring the resistance of a wire with an ammeter and voltmeter. 


In order to find out whether the current has any effect on resistance, 
add another cell and repeat the observations and calculations. 

Measure the diameters of the wires with a micrometer caliper and 
also ascertain the material of which the wires are made. 

1. Compare the resistance of two equal lengths of wire, of the 

*See Ammeters and Voltmeters in the appendix. 

[ 100 ] 



























Laboratory Manual of Physics 


101 


same material but of different sizes, and see if your results show 
that resistance varies inversely with the square of the diameter. 

2. Compare the resistances of equal lengths of wire, of the same 
diameter but of different materials, and give your conclusions 
regarding the importance of the material of which an electrical 
conductor is made. 

3. (a) What effect does the current in a conductor have upon 
its resistance? (b) How does the voltage drop over a conductor 
vary with the current? 

B. Resistance of conductors in series. Connect two of the wires 
in series so that the current passes through one after the other and also 
through the ammeter. Connect the voltmeter to read the potential dif¬ 
ference over both wires. Calculate the resistance from the ammeter 
and voltmeter readings and check the result against the sum of the 
resistances of the separate wires as found in A. 

Connect three of the wires in series and measure the total resist¬ 
ance. Compare with the sum of the resistances of the separate wires as 
found in Part I. 

4. Give the general law for calculating the total resistance of a 
number of resistances in series and show that it is confirmed by 
your experiments. 

5. (;a) Which is the same in all parts of a series circuit, volt¬ 
age or current? (b) What can be said about the possible posi¬ 
tions of an ammeter in a series circuit? 

C. Resistance of conductors in parallel. Connect two of the wires 
in parallel. When the resistance of wires in parallel is to be measured, 
the ammeter must be placed so as to read the total current through the 
wires. Connect the voltmeter so that it will read the potential drop in 
the wires. Record current and voltage and compute the resistance of 
this parallel circuit. Using the values of resistance of the separate wires 
found in A, above, calculate the resistance of this parallel circuit and 
compare your answer with the value obtained experimentally. 

6. Give the law for calculating the resistance of two branches 
in parallel and show that it is confirmed by your results. 

Connect three or four wires in parallel and measure their combined 
resistance. 

7. Give the general law for calculating the resistance of any 
number of branches in parallel and show that it is confirmed. 

8. Adding resistance in parallel has what effect on the total re¬ 
sistance? On total current? 

9. Adding resistance in series has what effect on the total re¬ 
sistance? On the current? 


102 


Laboratory Manual of Physics 


Suggested form of record 

A 


Wire 

No. 

Diameter and 
material of wire 

Potential drop 
or voltage 

V 

Current 

1 

Calculated 

resistance 

R 

Using one cell 


1 





2 





9 

O 




—:- - 

4 





5 





6 





Using two cells 

1 





2 





3 





4 





5 





6 







n 


Resistance of two wires in series: 

Voltage drop over wires No_and No_in series,-volts. 

Resistance of wires No_and No.._in series, (V/I) -ohms. 

Calculated resistance, using resistance from A -ohms. 

Resistance of three wires in series: 

Voltage drop over wires No_, No_and No_in series-volts. 

Current___amperes. 

Resistance of wires No. _, No. -—. and No- 

in series, ( V/I) _ ohms. 

Calculated resistance, using resistances from A _ ohms. 

G 

(Devise your own form of record.) 
















































































ADD. 7. THE WHEATSTONE BRIDGE 1 


Charles Wheatstone in 1843 invented an instrument called the Wheat¬ 
stone bridge which has a wide application in electrical measurements. 
Wheatstone bridges are made in several different forms but all depend 
upon the principle illustrated by the simple diagram of Fig. 51. 

Four resistances X, R, a and b are connected as indicated in the dia¬ 
gram. When the key k is closed, the current divides at C and one part, 
ii, flows through X and R to D and the other part, i 2 , flows through a and 
b to D. One end of the galvanometer G is connected permanently to E 

while the other end is con¬ 
nected to some point F, so 
chosen that no current 

flows through the galvano¬ 
meter. 

In the form of Wheat¬ 
stone bridge which we will 
use in this experiment, 

called the slide wire bridge, 
the resistances a and b are 
parts of a long straight 
wire of uniform cross-sec¬ 
tion. This wire is stretched between the points C and D, the point F being 
found by sliding the movable contact back and forth along this wire, 
until a point is found on the wire for which the galvanometer shows no 
deflection. 

When the point F has thus been located, the bridge is said to be 

balanced and E and F are at the same potential. Why? This means 

that the potential drop over the resistance X, which by Ohm’s law is 
equal to i x X X, fs exactly the same as the potential drop over the resist¬ 
ance a, which is equal to i 2 X 

As i\ X X = i 2 X cl, 

similarly p X R = U X b. 

Dividing the first equation by the second, 

A _ a 
R ~ b 

In the slide wire form of bridge, the resistances a and b are parts of 
the same uniform wire; consequently, these resistances are proportional 
to the lengths of wire CF and FD and the above equation becomes 

Unknown resistance length of wire adjacent to unknown resistan ce 
Known resistance length of wire adjacent to known resistance 


E 



Fig. 51. Diagram of a Wheatstone bridge. 


‘This experiment is substantially the same as Exp. Add. 6. except for the method of 
measuring resistance. 


[103] 







104 


Laboratory Manual of Physics 


Exp. Add. 7. Measure the resistance of several wires alone and when 
connected in series and parallel. 

A. Measurement of resistance with a Wheatstone bridge. Arrange 
and connect the apparatus’ as indicated in Fig. 52, and measure the re¬ 
sistance of a number of wires. 



X 

VVVj 

( c 

) ^ 

Res 

C 

■,/s tan 
Boa. 

p c 

cc 

p 

















r v/// ///X '///' 

■w- V 

i 

1 

C % 


E 

) F 


\ 

: d 


1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 






_J1 _ --- 



Fig. 52. Slide-wire Wheatstone bridge. 

Connect the unknown resistances at X, using connecting wires of 
as low a resistance as possible. If the resistance of the connecting wires 
is appreciable in comparison with that of X, it is a good plan to short 
circuit the connecting wires and measure their resistance. This may be 
subtracted from the values of X found by experiment, giving the true 
resistance of X. 

In order to balance the bridge, guess at the value of the unknown 
resistance and adjust the resistance box to this amount. If you have 
guessed correctly, the bridge will balance with the point F at about the 
middle of the wire. Make contact at the middle and note the direction 
of the deflection of the galvanometer. A very brief contact is usually 
sufficient. Try making contact at various points on the wire until you 
obtain a deflection in the other direction. Then find the point for which 
there is no deflection. The accuracy of the result is greatest when the 
point F is not too close to the ends. By changing the value of the known 
resistance in the box it is possible to bring the point F nearer the middle 
of .the wire. 


1 It is an easy matter to construct a serviceable slide wire bridge. Mount two binding 
posts a meter apart on a long board and stretch between them a meter length of No. 36 Ger¬ 
man Silver wire. (Any length will serve but a meter length is convenient). The heavy strips 
of metals shown in Fig. 52 are not necessary. Their purpose is to make it possible to use 
short connecting wires to the resistances X and R. If as short lengths as possible of No. 18 
copper wire, or larger, are used the arrangement shown in Fig. 51 will be perfectly satis¬ 
factory. 
































Laboratory Manual of Physics 


105 


1. If the balance point F is in the exact middle of the bridge 
wire, what is the relation between the unknown resistance X and 
known resistance R \? 

2. If the balance point F is found to be close to the end of the 
wire connected to the resistance box R, 'which is the smaller re¬ 
sistance, I or R1 

3. If the balance point F is found to be very close to the end of 
the wire connected to the resistance box R, should one increase 
or decrease the resistance in R, so as to make F come nearer the 
middle of the wire? 

B. Resistance of conductors in series. Connect in series two of 
the resistance wires used in Part I and place them at X. Measure their 
resistance in series and check the result against the sum of the resist¬ 
ances of the separate wires as found in Part I. 

Connect three or four of the wires in series and measure their total 
resistance. Compare your result with the sum of the resistances of the 
separate wires, as found in Part I. 

4. Give the general law for calculating the total resistance of a 
number of conductors connected in series and show that it is con¬ 
firmed by your experiments. 

C. Resistances of conductors in parallel. Connect two of the wires 
used in Part I in parallel and measure their combined resistance. 

5. Give the law for calculating the resistance of two conductors 
connected in parallel and show that it is confirmed by your results. 

6. Explain why the combined resistance of conductors in par¬ 
allel should be less than the resistance of either conductor alone. 

Connect three or four wires in parallel and measure their combined 
resistance. 

7. Give the general law for calculating the resistance of con¬ 
ductors in parallel and show that it is confirmed by your results. 

8. Adding resistance in parallel has what effect on the total re¬ 
sistance? 

9. Adding resistance in series has what effect on the total re¬ 
sistance? 


106 


Laboratory Manual of Physics 


Suggested form of record 


A 


Wire 

No. 

Size and kind 
of wire 

Distance adja¬ 
cent to X 

C F 

Distance adja¬ 
cent to R 

F D 

Known resist¬ 
ance in ohms 

R 

Unknown re¬ 
sistance in 
ohms 
* 

1 






2 






3 






4 






5 




. 


6 






B 

Measured resistance of wires No-and No- 

Calculated resistance of wires in series, using values from .4 


ohms. 

ohms. 


Measured resistance of wires No-- No. - 

and No_in series-ohms. 

Calculated resistance of wires in series, using values from A 


ohms. 


c 

Measured resistance of wires No. -, and No. -- in par¬ 
allel_ohms. 

Calculated resistance of wires in parallel, using values from A--ohms. 


Measured resistance of wires No.__-- No- 

and No__ in parallel-ohms. 

Calculated resistance of wires in parallel, using values from A 


ohms. 




































































ADD. 8. EFFICIENCY OF INCANDESCENT LAMPS 


When Edison invented the incandescent lamp with the carbon fila¬ 
ment he made possible the lighting of houses with electricity. Since then 
great progress has been made in increasing the efficiency of incandescent 
lamps. Two of the most important steps have been the use of filaments 
made of tungsten, which has a very high melting point and can be operated 
at a much higher temperature than carbon, and the use of gas-filled bulbs. 
The gas in a gas-filled bulb exerts a pressure on the filament and thus 
keeps it from evaporating so quickly. The filament can then be operated 
at a higher temperature than is possible in a vacuum lamp. In this ex¬ 
periment the old fashioned carbon lamp, operating at a temperature of 
about 1850° C. and giving a yellowish light, will be compared with modern 
tungsten lamps, operating at temperatures of about 2300° C. and giving 
a much whiter light. 

In order to measure the power required to operate a given lamp, it 
is necessary to know the current passing through the lamp and the po¬ 
tential drop. 

Power in watts = potential drop in volts X current in amperes. 

In order to compute the total amount of electrical energy used in a 
lamp it is necessary to consider the time during which the power has been 
used. The unit of electrical energy usually employed is the kilowatt- 
hour.. 


Energy in kilowatt-hours = 


power in watts X time in hours 
1000 



Fig. 53. Ammeter-voltmeter method of measuring 
resistance and power. 


Cost of operation per hour 
* kilowatts X cost per kilo¬ 
watt-hour. 

When the efficiency of an 
incandescent lamp is meas¬ 
ured in candle power per 
watt, the efficiency is given 
by the relation, 

. candle power 

Efficiency — - . ——- - 

power in watts 

The approximate candle 
powers of several types of 
lamps are given in the table 
at the end of this experiment. 

Exp. Add. 8. Measure the 
power necessary to operate 
various types of electric 
lamps. 


r io7] 














108 


Laboratory Manual of Physics 


Connect the apparatus as indicated in Fig. 53’, with the ammeter meas¬ 
uring the current through the lamp and the voltmeter the potential differ¬ 
ence, or voltage, applied to the lamp 1 . 

Measure the current and voltage for each lamp and calculate the 
power in watts. Record the observed data in the table at the end of the 
experiment and compute the data for the other columns. 

It is not expected that you will be able to test all the lamps listed. 
Some of the questions involve lamps which you will probably not test. 
In such cases use the information printed in the data table at the end of 
the experiment and make whatever computations are necessary to an¬ 
swer the questions. 

1. Compare the power required by a 32 c.p. carbon lamp and a 
40-watt vacuum tungsten lamp. Compare their candle powers. 
How much more efficient is the tungsten lamp than the carbon 
lamp? 

2. How many times more efficient is a 100-watt gas-filled tungs¬ 
ten lamp than a 25-watt vacuum tungsten lamp? 

Gas-filled lamps are not made in the smaller sizes since they are then 
not as efficient as vacuum lamps. This is because of the large amount of 
heat lost through convection and conduction when the bulbs are small. The 
heat losses become relatively less and less as the size increases, a 1000- 
watt gas-hileu lamp oemg twice as efficient as a 40-watt vacuum tungsten 
lamp. 

3. How many times more efficient is a 200-watt gas-filled lamp 
than a 100-watt gas-filled lamp? Than a 25-watt vacuum lamp? 

4. How much more light is given by a 200-watt gas-filled tungs¬ 
ten lamp than is given by five 40-watt vacuum lamps? 

5. Which is better for lighting a living room, one 200-watt gas- 
filled lamp in a properly designed fixture or six 40-watt lamps on 
a chandelier? Explain. 

6. Can you give a good reason why some dentists prefer to use 
cabbon lamps,, rather than tungsten lamps, to illuminate the 
mouth of a patient? 

7. What is the resistance of a 25-watt tungsten lamp ? Of a 40- 
watt tungsten lamp? Of a 16 c.p. carbon lamp? Of a 32 c.p. 
carbon lamp? 

8. Write down the simplest formula which can be used to com¬ 
pute the resistance of a lamp, if its voltage and wattage are 
known. 


^ee Ammeters in the appendix. 



Laboratory Manual of Physics 


109 


Suggested form of record 


Description 
of lamp 

P. D. 
in 

volts 

Current 

in 

amperes 

Power 

in 

watts 

Rated 

watts 

Rated 

candle 

power 

Efficiency 
in c. p. 
per watt 1 

Cost 

per 

hour 

16 c. p. carbon 





16 



32 c. p. carbon 





32 



25-watt tungsten 





18 



40-watt tungsten 





32 



60-watt tungsten 





oO 



75-watt gas-filled 
tungsten 





72 



100-watt gas-filled 
tungsten 





103 



200-watt gas-filled 
tungsten 





240 

i 



x The candle power listed is based on the operation of the lamp at rated watts and 
consequently, in computing the efficiency, the rated watts rather than the observed watts 
should be used. 


Special Experiment. School electric lighting system. Examine the 
main wires, the switch-box, the fuses and the branch wires. Trace and 
make a diagram of the circuit from the point where it enters the build¬ 
ing to the branch wires. 

Read the meter. Turn on more lights in the building and observe the 
effect on the meter. 

What is the purpose of the main switch? Why are the main fuses 
placed between the meter and the point where the circuit enters the 
building? 

What is the advantage of having a separate set of fuses for each 
branch circuit? Describe the type of fuse used. What is its ampere 
carrying capacity? 

Examine one of the branch circuits to see if it is overloaded. To do 
this, add up the number of amperes taken by all the lamps and appli¬ 
ances on the circuit. If this sum exceeds the rating stamped on the fuse, 
the fuse will blow. The number of amperes used by lamps operating 
at from 105 to 120 volts may be taken as follows: 25 watt, 0.13; 50 watt, 
0.43; 75 watt, 0.65; 100 watt, 0.87. The current used by electrical appli¬ 
ances can generally be learned by examining the name plate on the device. 

If convenient, place a low capacity fuse in one of the branch circuits 
and try the effect of overloading the circuit. 














































ADD. 9. ELECTRICITY IN THE HOME 


In this experiment we will study the power used by heating devices 
and motors and will determine the efficiency of some form of heating 
device. 

When electrical energy is transformed into heat, every watt-hour of 
electrical energy produces exactly 864 calories of heat. The question of 
whether this heat is available for the purpose for which you wish to use 
it is a matter which depends upon the efficiency of the device. For 
example, a great deal of the heat from an electric toaster is wasted in 
the room. On the other hand an electric heater, made to heat a room, 
is 100 per cent efficient, for all the heat remains in the room. An im¬ 
mersion coil for heating water is nearly 100 per cent efficient, for the 
heating element is almost completely surrounded by the water. 

The practical efficiency of a heating device is the fraction of heat 
produced which is available for the intended purpose. 


Practical efficiency 


useful heat produced 
total heat produced 


It is not always an easy matter to determine the practical efficiency, 
due to the fact that it is often difficult to measure the useful heat produc¬ 
ed. Fortunately the total heat produced is easily determined, not by di¬ 
rect measurements of the heat, but from a knowledge of the fact that 

Heat in calories = volts X amperes X time (seconds) X 0-24, 

= watts X seconds X 0.24, 

«*». watt-hours X 864. 


Exp. Add. 9, Part I. Measure the power required by various house¬ 
hold appliances. 

Connect each of the applicances, in turn, as indicated in Fig. 53. 
Place the ammeter so that it reads the current which passes through the 
device being tested and connect the voltmeter outside the ammeter. 1 

Read amperes and volts and record the data in a table similar to that 
at the end of this experiment. Compute the power in watts and the cost 
per hour, using the local rate. 

1. Compare the power consumed by the various applicances. 
Which are more expensive to operate, household electric motors 
or household electric heating appliances? 

2. How does the power required to operate a flatiron compare 
with that needed to light the living room in your house? 

3. How does the power required to operate an electric fan com¬ 
pare with that used by a 25-watt lamp? With that used by an 
electric iron? 


1 Se« Ammeters in the appendix. 


[HO] 




Laboratory Manual of Physics 


111 


Exp. Add. 9, Part II. Determine the 'practical efficiency of some 
form of water heater. 

This experiment may be performed with a dish of water placed on a 
hot plate, or with water contained in the dish of a small electric stove or 
chafing dish. At least one group of students should test an immersion 
coil, if one can be secured. 

Connect the apparatus in such a way that the current is measured 
by the ammeter and the potential drop by the voltmeter. Weigh or 
measure an amount of water suitable to the container and measure its 
temperature t x . Place the water on the hot plate and turn on the current 
on an even minute, nothing the time T x on a watch. Stir the water gently 
from time to time and, when its temperature is about 90°C., turn off the 
current on an even minute and note the time To. Stir the water and ob¬ 
serve its temperature t 2 . 

Compute the watt-hours of energy consumed and the number of 
calories of heat actually delivered to the water. Then compute the prac¬ 
tical efficiency of the applicance, including the dish, in calories per watt- 
hour. Also compute the efficiency in per cent of the maximum possible 
efficiency, which is 864 calories per watt-hour. 

1. Explain fully why an immersion heater is a more efficient ap¬ 
pliance for heating water than is a hot plate or chafing dish. 

2. Why does a flatiron require a relatively large amount of heat? 

3. Discuss the effect of the dishes used with an electric stove 
upon its efficiency, considering among other factors, (a) shape, 

(b) material, (c) size. 


Suggested form of record 

Part I. Power Required by Appliances 


Description of 
appliance 

Current in 
amperes 

P. D. 
in volts 

Power in 
watts 

Rated 

watts 

Cost per 
hour 














































112 


Laboratory Manual of Physics 


Part II. Efficiency of Appliances 



Appliance 
No. 1 

Appliance 
No. 2 

Appliance 
No. 3 

Description of appliance 


Current in amperes. / 




Potential difference in volts. V 




Power in watts. VI 




T ine of closin'? switch. Ti 




Time of opening switch, T< 




Total time in hours. T 




Enemy in watt-liours. VIT 




Cost of operating, per hour 




Mass of water in grams, m 




Initial temperature, ti. in °C. 




Final temperature, U, in °C. 




Increase of temp, of water in °C., t« — ti. 




Heat delivered, in calories, mX(L — fi)Xl. 




m 1 U — t \) 

Efficiency in calories per watt-hour, — — 

• 



Efficiency in percent of ideal or maximum 
possible efficiency. 

































































































ADD. 10. THE BUN,SEN PHOTOMETER. 


Before attempting to work with the photometer, the student should 
read the introduction to Exp. 34. 

The Bunsen photometer is sometimes called the grease spot photo¬ 
meter, because it consists of a screen having in the middle of it a spot 
which is greased to make it partly transparent. If both sides of the screen 
are equally illuminated, this spot tends to disappear. 



When the candle power of a source of light is measured with this 
photometer, use is made of the principle that the candle powers of two 
sources of light which produce equal brightness on the photometer screen 
are directly proportional to the square of the distances of the sources from 
the screen. It is this principle which we will test in this experiment. 


Exp. Add. 10. Construct a Bunsen photometer and use it to test the 
relation between the candle powers of two lights and their distances from 
the photometer screen. 

In a darkened room, set up a Bunsen photometer 1 similar to the one 
shown in Fig. 54. 

Place a single candle on each side of the screen. If necessary, trim 
the wicks to make them burn equally, and then move the screen back and 
forth on the meter stick Until the grease spot either disappears or else ap¬ 
pears as nearly as possible the same on both sides. Measure and record 
the distance of each candle from the spot on the screen. 

Now place two candles on one side of the screen, leaving the single 
candle on the other side. Make sure that all three candles are burning 
properly and again adjust the position of the screen until the spot again 
disappears or has the same appearance on both sides. 

Repeat with three, and finally four candles on one side, and a single 
candle on the other side. 

For each of the above four cases, compute the ratio of the candle 
powers of the two sources and also the ratio of the squares of their re¬ 
spective distances from the screen. 


lr To make a Bunsen screen, put a drop of melted paraffin in the middle of a piece of 
unglazed white paper and heat it until the spot is about a centimenter m diameter and 
transparent. In an 8 cm piece of heavy card board, cut a 4 cm hole and paste the paper 
over the hole with the grease spot in the middle. 

[ 113 ] 





114 


Laboratory Manual of Physics 


1. State in your own words and also in symbols the principle re¬ 
vealed by a comparison of these ratios. 

2. Why does the spot tend to disappear when both sides are 
equally illuminated? 

3. If a light is moved four times as far away from an object, 
how much brighter must the light be made to illuminate the object 
to the same degree as before? 

4. Explain how your photometer can be used to measure the can¬ 
dle power of an incandescent lamp. 


Suggested form of record 


Number of 
candles, 
left source 

Distance of 
left source 
from screen 
L 

Number of 
candles, 
right source 

Distance of 
right source 
from screen 
R 

C.P., left source 

L 2 

RT 

C.P., rt. source 




' 

I 



Special Experiment. Use a Rumford or Bunsen photometer to 
measure the candle power of an incandescent lamp, a gas burner or other 
source of light, following the directions given in Exp. 34 and Exp. Add. 10. 

If an incandescent lamp is tested, first measure its candle power 
when it is in a vertical position, with the light from the side of the bulb 
falling on the screen; then when it is in a horizontal position, with the 
end of the bulb pointing toward the screen; and finally, when a shade is 
placed behind it. In each case make several trials and compute the aver¬ 
age candle power. Compare the results for the several cases. 

















APPENDIX 

AMMETERS. An ammeter is an instrument used to measure the current in an 
electrical circuit. 

In a series circuit an ammeter may he placed at any point, since the current is the 
same in all parts of such a circuit. In the case of a parallel circuit, the ammeter may be 
placed in one of the branches to measure the current in the branch, or it may be placed 
in the main circuit to measure the total current. 

Ammeters are made with a very low electrical resistance; in most cases the effect 
of their resistance may be neglected. Since an ammeter has such a low resistance, it is 
easily “burned out” and it must not be connected to a source of current unless sufficient 
outside resistance has been placed in series with it. If a circuit contains an ammeter, 
it is good practice to close the switch for only an instant at first, in order to see whether 
or not the current is too large to be read by the instrument. 

Measurement of small currents unth A. C. ammeters. You will find that the scales 
of alternating current ammeters are not uniform and that small currents cannot be 
read on them with accuracy. To secure a higher degree of accuracy in reading small 
currents on an A. C. ammeter, use the following method: Connect in the circuit some 
electrical resistance, such as an incandescent lamp, which will give a readable deflection 
of the ammeter; read and record this deflection. Then connect the device in which you 
are interested in parallel with the first resistance, so that its current is added to that 
of the latter; again read the ammeter. The difference between these two readings is 
the required current. 

BALANCES. Beam balances and platform balances are instruments used to com¬ 
pare masses. 

Before using the beam or platform balance, always make the following initial ad¬ 
justments: (1) Observe whether 

the balance swings freely; (2) If 
the base is provided with leveling 
screws, adjust these until the base 
is horizontal; (3) Slide the weight 
on the graduated beam until it is 
in the zero position; (4) Adjust 
the beam until it will balance 
evenly when the pans are dry 
and clean. 

To measure the mass of an 
object, place the object on the left 
pan of the balance and counter¬ 
balance with weights placed in the 
right pan. Try out the larger 
weights first. Determine the 
largest single weight and then add 
smaller weights to the pan in suc¬ 
cession. When the object has 
been balanced to within 10 grams 
of its mass make the final ad- 



Fig. 55. Beam balance. 


[ 115 | 
































116 


Laboratory Manual of Physics 


justment by sliding the weight, attached to the graduated beam, to the right. I>o uot 
wait for the oscillations of the balance to cease entirely; if the pointer moves thiough 
approximately equal small distances on both sides of the zero-point on the pointer scale, 
the weighing is sufficiently exact. 

The mass of an object weighed in the above manner is the sum of the weights on the 
pan plus the reading of the left edge of the weight on the graduated beam, since each 
small division on the graduated beam represents one-tenth of a gram. The mass should 
be recorded in grams and a decimal fraction thereof. 

Return weights to their proper place as soon as you have tinished weighing. Leave 
the pans dry and clean. 

Spring balances. A spring balance is a device used primarily for the measurement 
of force. The scale of a spring balance is so made that, when the balance is hung in a 
vertical position, it will give correctly the weight of an object hung from its hook or the 
amount of any other force acting vertically downward on the hook. If. however, a spring 
balance is poorly constructed or has been misused, it may not give a correct reading 
even when held in the vertical position for which it was made. Consequently, it is 
important always to test a balance before using it. Such a test is made by hanging 
the balance in a vertical position, with the hook hanging free and without a load on 
it. Under these conditions the pointer should read zero. If the reading is differ¬ 
ent from zero, the difference, called the “zero reading," should be read and recorded. A 
“zero reading” which is less than zero must be added to all future readings taken with 
the particular balance in question; if. on the other hand, the “zero reading” is greater 
than zero, the correction must be subtracted from future readings. 

In measuring a force, it is often necessary to hold a spring balance horizontally, 
at an angle with the horizontal, or even in an inverted position. In any of these cases 
the hook and bar of the balance are not exerting their entire weight against the spring. 
In fact, when the balance is held horizontally, the spring of the balance is not supporting 
the weight of the hook and bar at all. while in the case of the inverted balance, the hook 
and bar are actually compressing the spring. It will be found, for example, that a 2000 
g spring balance held in a horizontal position will read about 25 g less than the true 
reading, this being due to the fact that the hook and bar of such a balance weigh about 
25 g. 

Thus it can be seeu that a “zero reading” should always be taken for the particular 
position in which a spring balance is to be used and that this reading must be added 
or subtracted, as the case may be, from all subsequent readings taken with the balance 
in this particular position. 

Another way of finding the correction to be applied in the two cases where the bal¬ 
ance is used in a horizontal or in an inverted vertical position is to hang from the bal¬ 
ance a second balance like it, the latter one being hung in an inverted position. The 
correction for a horizontal position of the balance is then half the difference of the two 
balance readings, while the correction for an inverted position is the entire difference be¬ 
tween the two readings. 


CALIPERS. Micrometer Caliper. The fixed scale of a micrometer caliper is gradu¬ 
ated in Millimeters and the movable scale is so ‘graduated that the barrel moves through 
one hundredth of a millimeter when turned through one division. When the edge of 
the movable barrel stands exactly on a millimeter line of the fixed scale, the zero mark 


Appendix 


117 


on the barrel coinciding with the line, the opening between the jaws is a whole number 
of millimeters. For example, the reading on the instrument shown in Fig. 5G is exactly 
6 mm. It will he noted that in this case the zero mark on the movable scale falls exact¬ 
ly on the line running lengthwise on the lixed scale. 

One complete turn of the barrel moves it through one-half millimeter (50/100 mm). 
Hence, if the edge of the barrel is less than half-way between two marks on the fixed 

sea'e. the opening between the jaws is a 
whole number of millimeters plus the num¬ 
ber of hundredths of millimeters read on 
the movable scale. If the edge of the barrel 
is more than half-way between two marks 
on the fixed scale, a fact easily determined by 
the eye, the opening between the jaws is 
a whole number of millimeters plus 0.50 mm 
plus the number of hundredths of millimeters read on the movable scale. 

In vising a micrometer caliper it is first necessary to obtain the zero reading. Care¬ 
fully close the jaws by turning the milled head on the extreme right, Fig. 56. If the 
instrument is provided with a ratchet stop, a clicking sound will be heard when the jaws 
make contact. If there is no ratchet head, hold the milled head lightly so that it will 
slip between the fingers when the jaws make contact. W hen the jaws are closed the 
zero mark of the movable scale should fall exactly on the line running lengthwise on the 
fixed scale and the edge of the movable barrel should rest exactly on the zero mark of 
the fixed scale. If this is not the case, determine the number of hundredths of a milli¬ 
meter which this reading differs from zero; this correction must be added or subtracted, 
as the case may he. from subsequent readings made with the instrument. 

To measure the thickness of an object, open the caliper jaws wide, insert the object 
between the jaws and again turn up the milled head until the jaws make contact with 
the object. Read the whole number of millimeters and half millimeters on the fixed 
scale and add the number of hundredths of a millimeter read on the movable scale. 
Finally, apply to this reading the zero correction. 




Vernier caliper. The vernier caliper is equipped with a vernier scale. The vernier 
s a device for measuring the fractional parts of a scale division, thus making it unneces¬ 
sary to estimate parts of a scale division, as has to be done in reading a meter stick. 

The vernier caliper shown in Fig. 57 can be read to 0.01 cm or 1/1-8 in. It is 
'quipped with two sets of jaws. The lower set of jaws is used for making outside meas¬ 
urements, such 
as the diameter 
of a sphere or of 
a rod. The up¬ 
per set of jaws 
is for making 
inside measure¬ 
ments, such as 
the inside diam¬ 
eter of a piece of 
pipe. 

The narrow rod on the extreme right of the instrument, Fig. 57, is a depth gauge. 

A vernier caliper has two scales, a fixed scale and a movable vernier scale. The 



















118 


Laboratory Manual of Physics 


fixed scale is usually graduated on the upper edge in inches and sixteenths of an inch, 
and on the lower edge in centimeters and millimeters. 

To use a vernier caliper, place the object to be measured between the jaws and press 

the jaws firmly against it. Observe the left- 
hand division on the fixed scale which is 
nearest to file zero mark on the sliding scale. 
(In Fig r»S this is 3.1 cm). Then observe the 
mark on the sliding scale which lies in the same 
straight line as some mark on the fixed scale 
(In Fig. HS this happens to be the fourth line). 
This division on the sliding scale which thus 
coincides with a division on the fixed scale gives 
the fractional parts of the smallest scale division, 
Fig. 58. Verner scale ; reading is 3.14 cm. or the hundredths of a centimeter, which must 
he added to the reading on the fixed scale. (Thus the complete reading on the scale 
shown in Fig. 58 is 3.14 cm). 



DENSITIES. The densities of the following substances are given in grams per cubic 
centimeter: 


Alcohol _ 

Aluminum _ 

Brass_,_ 

Carbon bisulfid, at 22° C 

Coal, anthracite_ 

Coal, bituminous_ 

Copper - 

Gasoline _ 

Glass, crown_ 

Glass, flint _ 

Glycerin, at 20° C_ 

Gold, IS carat_ 

Gold, pure___ 

Granite_ 

Hydrochloric acid _ 

Ice _ 

Iron_ 

Kerosene _,_ 

Lead _ 

Marble _ 


_0.S1 

_2.70 

_S.4-8.7 

_1.26 

__1.4-l.S 

_1.2-1.5 

_8.93 

0.74-0.76 


_2.9-4.S 

_1.26 

_14.9 

_19.3 

2.6-2.S 

_1.27 

_0.92 

__7.1-7.8 

0.78-0.80 

_11.3 

_2.6-2.S 


Mercury, 0° C_13.595 

Mercury, 20° C_13.546 

Milk_1.028-1.035 

Nickel -8.9 

Platinum_21.4 

Silver__ _— 10.5 

Tin ___— _7.3 

Water: 

Sea, at 15° C_1.025 

Pure, at 4° C_1.000 

Pure, at 100° C_0.958 

Woods: 

Cedar ___0.5-0.6 

Cherry_0.7 0.9 

Chestnut_0.5-0.6 

Ebony _1.1-1.3 

Mahogany_0.6-0.9 

Oak _0.6 0.9 

Pine_0.4-0-7 

Zinc_7.1 


1 

cm 

= 0.394 

in. 

1 

in. = 2.54 

cm. 

1 

m 

= 39.37 

in. 

1 

ft. = 30.5 

cm. 

1 

km 

= 0.621 

mi. 

1 

mi. = 1.61 

km 

1 

kg 

= 2.20 

lb. 

1 

oz. = 28.4 

or 

to* 

1 

1 

= 1.06 

qt. 

1 

lb. = 454 

or 

to* 


EQUIVALENTS. 






































































Appendix 


119 


GALVANOSCOPES. A galvanoscope, as is clearly shown in Fig. 28, consists of a 
coil or coils of wire which encircle a magnetic needle. When in use, the frame must be 
placed in such a position that the plane of the coil of wire is parallel to the needle, and 
the compass case should be turned until the needle reads zero. In order to keep the in¬ 
strument in position it is a good plan to clamp the galvanoscope frame to the table, or 
to weight it down with books, blocks, or other non-magnetic material. 

Galvanoscope frames usually have several coils with different numbers of turns. The 
more times the current passes around the needle the greater will be the deflection. For 
this reason the coil with the largest number of turns is used when the currents are weak. 

No matter how strong the current may be, the needle will never deflect more than 
1)0 degrees from zero. From this fact one can see that the deflections of the needle are 
not proportional to the current. In the case of small deflections, however, say less than 1 
25 or 80 degrees, the deflections are nearly proportional to the current. It is partly for 
this reason that the studept is urged, in many of the experiments, to introduce enough 
resistance in the circuit to bring all the deflections within this range. 

GRAPHS. The graphical method is a very important way of making clear the re¬ 
lation between the quantities involved in an experiment and it should be used wherever 
possible. If the student is not acquainted with the graphical method, he should consult 
a text-book on algebra. 

In many experiments the variable quantities involved are only two in number and, 
wdien a change in one of these quantities is accompanied by a change in the other, the 
one quantity is said to vary with the other. The way in which this variation occurs 
often can be learned from the plotted curve, or graph. 

In plotting and readng curves, the following facts will prove useful: 

(1) The origin O, which is the intersection of the horizontal X-axis and the vertical 
Y-axis, is generally placed in the lower left-hand corner of the page of cross-section 
paper (see Exp. 17, Fig. 20). In some cases, however, it is necessary to place the origin 
off the page and, in other cases, as in Exp. 12, Fig. 14, near the center of the page. 

(2) In plotting curves it is not necessary that the same numerical value be as¬ 
signed to a scale division on the horizontal and vertical axes. It is usual to choose values 

for the scale divisions so that the curve will just about till the page. The scales chosen 
should be clearly indicated on the paper. 

(3) A curve drawn through a series of plotted points should be a smooth curve. 
This curve should be drawn in such a manner that it passes as close as possible to the 
plotted points, leaving about half the points on either side of the line; this is the graph¬ 
ical way of averaging. It is not possible to draw a smooth curve through all the points, 
when these points represent the results of an experiment; this is due to the fact, that 
the results obtained by repeating the same measurement differ slightly from one another, 
owing to unavoidable errors in making the readings and to imperfections in the instru¬ 
ments. One of the great advantages of the graphical method is that a serious error in 
experimentation is at once made evident by the fact that the point so obtained does not 
lie near the smooth curve which can be drawn close to the remaining points. 

(4) When two quantities vary in such a way that their ratio, or quotient, is a 

constant, one of these quantities is said to vary directly as the other; any graph repre¬ 

senting a direct variation must be a straight line and must pass through the origin. 
When, on the other hand, two quantities vary in such a way that their product is a 
constant, one of these quantities is said to vary inversely as the other; a curve repre- 


120 


Laboratory Manual of Physics 


senting an inverse variation will always be of a tjpe known as a hypabola. Besides 
direct and inverse variations there are other ways in which two quantities maj ' ai J> 
and for each kind of variation there is a typical kind of curve. 


HUMIDITY 

(a) Vapor Pressure and Mass of Water A apor in Saturated Air 


Temperature 
in ° C 


-40 

— S 

— 6 
— 4 
_ 2 

0 

*> 

4 

0 

8 

10 

12 


Pressure in 
mm of mercury 


2.2 

2.5 

2.9 
3.4 

3.9 

4.6 
5.3 
6.1 
7.0 
8.0 
9.1 

10.4 


Mass in grams 
per cubic me.er 


2.2 

2.6 

3.0 

3.5 
4.1 
4.8 

5.6 
6.4 

7.3 

8.3 

9.4 
10.7 


Temperature 
in ° C 


14 

16 

18 

20 

->•> 

24 

26 

28 

30 

32 

34 

36 


Pressure in 
mm of mercury 


11.9 

13.5 

15.3 

17.4 

19.6 
22 2 
25.0 
28.1 

31.5 
35.3 

39.5 
44 2 


Mass in grams 
per cubic meter 


12.1 

13.6 
15.4 

17.3 

19.4 
21.8 

24.4 
27.2 

30.4 
33.8 

37.6 

41.7 


(b) 

Relative Humidity 

from 

Wet and 

Dry Bulb Thermometer 

Readings 


Dry bulb 
thermometer 



Difference between dry 

and wet bulb 

thermometer readings 


°C. 

1° 

9 ° 

3° 

4° 

5° 

6° 

7° 

8° 

9° 

10° 

n° 

12° 

—12 

62 

7 

_ 

-- 

-- 








— 9 

70 

39 

9 

-- 

-- 

-- 

-- 

-- 

— 

-- 

-- 

— 

- 6 

74 

49 

25 

2 

-- 

— 

-- 





-- 

- *_> 

7S 

57 

36 

17 

-- 

— 

-- 






0 

81 

64 

46 

29 

13 

— 

-- 

-- 

-- 

-- 

— 

— 

•> 

o 

S4 

69 

54 

40 

25 

12 

— 






6 

87 

73 

60 

47 

35 

23 

11 






9 

88 

76 

65 

53 

42 

•>9 
• )— 

•)•> 

12 

• > 

O 

-- 

-- 

-- 

12 

89 

78 

68 

58 

48 

38 

30 

21 

12 

4 

-- 

— 

15 

90 

80 

71 

62 

53 

44 

36 

28 

20 

13 

4 

-- 

18 

90 

82 

73 

65 

57 

49 

42 

35 

27 

20 

13 

6 

21 

91 

S3 

75 

67 

60 

53 

46 

39 

32 

26 

19 

13 

24 

92 

85 

77 

70 

63 

56 

49 

43 

37 

31 

26 

21 

27 

93 

86 

79 

72 

65 

59 

53 

47 

41 

36 

31 

26 

30 

93 

86 

79 

73 

67 

61 

55 

50 

44 

39 

35 

30 

33 

93 

86 

SO . 

74 

6S 

63 

57 

52 

47 

42 

37 

33 

36 

93 

S7 

81 

75 

70 

64 

57 

54 

50. 

45 

41 

36 








































Appendix 


121 


The coefficient of 'linear expansion of a substance is the 


LINEAR EXPANSION, 
fractional change of length 


Aluminum _ 0.000023 

Brass_ 0.000019 

Copper- 0.000017 

German silver _ 0.000018 

Glass_ 0.00008 


per degree Centigrade. 


Gold- 0.000014 

Hard Rubber ___ 0.000069 

Ice _ 0.000069 

Irin (cast) _ 0.000010 

Lead _ 0.000012 


Marble_ 0.000012 

Nickel_ 0.000013 

Platinum _ 0.00009 

Quartz glass_ 0.0000003 

Silver _ 0.000019 



m — 





- M 


MANOMETERS. The manometer is an instrument for measuring fluid pressure. 
There are several types of manometers, but the only one which we will consider is the 
open-tube form. 

The open-tube manometer is used when the pressures are not excessive. It consists 
of a piece of glass tubing, open at both ends and bent into the 
shape of a U. A meter stick is placed upright between the two 
arms of the U-tube, so that the difference in levels of the liquid 
in the two arms can be measured, (Fig. 59). This liquid is gen¬ 
erally mercury, though for rather small pressures, oil, gasoline 
or water is often used. 

Before a pressure is applied, the levels of the liquid in the 
two arms are at the same height, since the air presses down 
equally on both liquid surfaces. If, however, the pressure on 
m. Fig. 59, is increased, a difference of levels h results, which 
depends only upon the added pressure and upon the liquid used 
in the tube. The size of the tube will have no effect on the dif¬ 
ference of levels in the two arms. Hence the cross-section of the 
tube may be thought of as being 1 square centimeter, and the 
pressure may be found by calculating the pressure in grams per 
square centimeter of a column of the manometer liquid equal in 
height to the difference in levels h. 

The pressure so found is called the gage pressure, and it is 
given by the formula 

1 o = h X o’. 

When li is in centimeters and the density d is in grams per 
cubic centimeter, the pressure p is in grams per square centi- 

59. Open-tube 
manometer. meter ‘ 


Fi 


RESISTANCE. The specific resistance of a substance is the electrical resistance 
in ohms of one centimeter length of a cylinder of the substance having a cross-sectional 
area of one square centimeter. 


Aluminum _ 2.7x10 — e 

Brass _ 7.0x10 — 6 

Constantan (6% Cu, 40% Ni.)— 49.0x10— 6 

Copper _ 1.7x10 — c 

German silver _ 33.0x10 — 6 

Gold - 2.0x10— 6 

Iron _ 10.0x10— 6 

Lead ___ 20.0x10— 6 


Magnesium _ 42.0x10 — 6 

Mercury _ 94.0x10 — 6 

Nickel _ 7.0x10— 6 

Platinum _ 10.0x16— 6 

Silver _ 1.5x10 — 6 

Tungsten _ 6.0x10— 6 

Zinc _ 6.0x10— 6 

































122 


Laboratory Manual of Physics 


SPECIFIC HEAT. The following specific heats are given in calories per 
per degree Centigrade. 

Aluminum -0.2 Iron - 


Brick_°- 2 Lead - 

Copper _0.09 Mercury . 

Earth _0.2 Nickel — 

q 0 P 1_0.03 Soapstone 

Glass_ 016 zinc 

Ice--0.5 


gram 

_ 0.11 
_ 0.03 
_ 0.03 
_ 0.1 
_ 0.2 
_ 0.09 


USEFUL CONSTANTS AND FORMULAS. 

1 cm 3 of water at 4°C has a mass of 1 g. 

1 cu. ft. of water at 4°C has a mass of 62.4 lbs. 

1 atmosphere of pressure = 1033.6 g per cm 2 . 

1 atmosphere of pressure — pressure of (6 cm of meicury. 

7T = 3.1416 y 2*= 1.414 
tt 2 = 9.8696 V 3 = 1.732 
Circumference of circle = 2-nr = Trd. 

Area of circle = -nr 2 = i^d 2 . 

Surface of sphere = 4-Trr 2 = 7rd 2 . 

Volume of sphere = --- 7rr 3 = J-n-d 3 . 

O 

Volume of right cylinder = 7rr 2 l = i^rd 2 !. 


VOLTMETERS. Voltmeters are similar to ammeters in their internal construction 
except that they have a high resistance. The current which passes through them is 
equal to the voltage divided by the resistance of the instrument. Consequently the 
reading of a voltmeter is proportional to the voltage applied to its terminals. Voltmeters 
are always connected in parallel with that part of a circuit over which the potential 
drop is to be measured. 

Position of Voltmeters. If a voltmeter is connected around an ammeter, as in 
Fig. 53. the voltmeter reading includes not only the potential drop in the circuit but 
also the drop in the ammeter. This, of course, introduces an error, but with alternating 
current voltmeters it is usually much smaller than the error which would result if 
the voltmeter were connected directly to the lamp terminals. The reason is that A. C. 
voltmeters take a considerable amount of current, sometimes as much as 0.1 ampere; 
consequently, if the ammeter reading is small, this additional current introduces an 
appreciable error. 

In Fig. 50 the voltmeter is shown connected inside the ammeter, with the ammeter 
reading not only the current in the circuit but also the current taken by the voltmeter. 
This form of connection is permissible with D. C. voltmeters since they seldom require 
a current of more than .02 ampere. Either form of connection may be used with D. C. 
voltmeters but connections as shown in Fig. 53 are usually to be preferred when A. C 
instruments are used. 















LIST OF APPARATUS 

The second column, under “Apparatus," contains a complete list of the apparatus reuqired for each 
of the experiments in this manual. In the third column, under ‘‘suggestions," substitute apparatus is 
suggested and home-made equipment is described. 


Exp. No. 

Apparatus 

Suggestions 

1-1 

Beam or platform balance. 


* 

Set of slitted iron weights, 10-500g. 

Meter stick. 



3 rectangular blocks of different 
kinds of woods. 


l-II 

Steel or glass ball, % in. 

Micrometer caliper, with rachet. 

Beam or platform balance and set 
of weights, Part I. 

Meter stick, two blocks and an ad-i 
ditional ball. 

2-1 

Aluminum cylinder, 7.5x2.5 cm, with 
hook. 

A spherical oy rectangular piece of 
metal, about 40 cm. 3 


Battery jar, 5x7 in. 

Beaker, Exp. 17, or calorimeter 
vessel, Exp. 15. 


Thread. 



Balance, Exp. 1, fitted with a sup¬ 
port for the above jar. 

Spring balance, Exp. 0. If the beam 
balance of Exp. 1 is not fitted, 
with a jar support, use a home¬ 
made wooden bench, Fig. 3. 


Vernier caliper or the micrometer 
caliper of Exp. 1. 

Meter stick and two blocks, Exp. 1. 

2-11 

Brass weight, coal, rock, etc. 
and thread of Part I. 



Balance, weights, jar support, jar 
and thread of Part I. 


3-1 

Constant weight hydrometer tube, 
50 cm. 



Glass tube, 110x4cm, with rubber 
stopper. 

Clamp for above tube. 

May be omitted. 


Tripod base, rod and right angle 
clamp. 

Rubber bands. 

The length immersed in the liquid 
is more conveniently measured on 
a paper metric scale placed in¬ 
side the tube. 


Theremometer, 10 to 110° C- 



Universal hydrometer. 

May be omitted. 


Gasoline. 



Lead shot. 



Meter stick, Exp. 1. 

1 


[123] 

























124 


Laboratory Manual of Physics 


Exp. No. 

Apparatus | 

Suggestions 

3-11 

Water proof wooden block. 

Lead sinker with hook. 

Balance, support and weights, 
Exp. 2. 

Battery jar, Exp. 2. 

Thread, Exp. 2. 

Large piece of wax. paraffin or cork. 

A balance weight. 

'Spring balance, Exp. 6. 

Beaker or calorimeter cup. 

4 

Boyle’s law tube, J-tUbe form, 100 
and 20 cm arms. 

Support for above tube. 

Barometer. 

Mercury, 1 lb. 

Thermometer, Exp. 3; meter stick. 

• 

5-1 

Alcohol, denatured. 

Constant weight hydrometer tube, 
Exp. 3. 


5-11 

2 tumblers. 

Hydrochloric acid. 

Use tumblers of demonstration cell. 
Exp. 24, or beakers, Exp. 17. 

6 

2 spring balances, Hat back 2000 g. 
2 weight hangers. 

Fish line or strong cord. 

Metgr stick and weights, Exp. 1. 

Bags tilled with sand or shot may 
be substituted for the weight 
hangers and weights. Each 

weight should be a little less than 

1 kg. 

7-1 

Weight, about 1.5 kg. 

Support for spring balances. 

Two spring balances and cord, 
Exp. 6. 

Bag tilled with sand or shot. 

Hang from nails hi wall or from 
table crossbar. 

7-II 

Spring balance, flat back, 2000 1 g. 

Three "Stone” tension clamps or a 
force board. 

Dividers or pencil compass. 

Ruler, metric and English. 

Two spring balances. Exp. 6; 
wooden block. Exp. 1 or 14; cord 

Note. Part II may be performed 
with the apparatus of Part I. The 
weight L must be known and the 
supports A and B should be placed 
far enough apart to stretch the two 
balances to about half their full 
range. A smooth vertical board, on 
which to fasten the notebook paper, 
should be placed behind the cords. 







































Apparatus 


125 


Exp. No. 

Apparatus 

Suggestions 

8 

Pendulum clamp, wood. 

Ball, iron, 2.5 cm, drilled. 

Ball, hard wood, 2.5 cm, drilled. 

Watch with second hand. 

Magnifying glass. 

Meter stick and thread. 

A nail in the wall, or a split cork 
held in a burette clamp or fitted 
into a hole in a board. 

Steel ball, Exp. 1, and sealing wax. 
Used only in Part I. 

Use reading glass, Exp. 35, or linen 
tester, Exp. 36. 

9-1 

Stop watch or watch with second 
hand. 

Cord, tine quality, about 75 ft. 

Glass awning ring or a metal ring. 

Drilled iron ball, Exp. 8, or other 
weight. 

Meter stick. 

Note. A wire with hook and turn- 
buckle, on which runs a car equipped 
with cone-bearing pulleys, can he 
purchased. Some instructors prefer 
to use a grooved inclined plane and 
steel ball for this experiment. Both 
of these forms of apparatus can be 
used indoors. 

9-11 

Stop watch, meter stick and long 
cord. 


10 

2 single pulleys. 

Support for pulley. 

Fish line or cord. 

Weight hanger and spring balance, 
Exp. 6; weights; meter stick. 

Hang from table crossbar or from 

ring stand. 

Note. The experiment may be con¬ 
tinued with more than two pulleys, 
but time will be saved and less ap¬ 
paratus required if the instructor 
demonstrates such additional experi¬ 
ments in the presence of the class as 
a group. 

11-1 

Empty wooden crayon hox. 

Smooth board, about 120x15 cm. 

Spring balance, Exp. 6. 

Cord, Exp. 6; weights. 

A wooden block. 

Inclined plane, Exp. Add. 3, or the 
top of the table. 

If desired, substitute for the spring 
balance: pulley, Exp. Add. 3; 
weight hanger, Exp. 6: balance, 
Exp. 1. 

Note. It is more satisfactory to use 
paper for the two surfaces in contact. 
Fasten large sheets of paper to the 
board and also around the box. 

11-11 

Same as above plus sheets of tin, 
brass, etc. 





























126 


Laboratory Manual of Physics 


Exp. No. 

Apparatus 

Suggestions 

1 

12 

Steam generator. 

• 


Bunsen burner or alcohol stove. 



Co-ordinate paper, millimeter ruled. 
Ice or snow. 



Tumbler, Exp. 5. 

Calorimeter cup, Exp. 15. 

13-1 

Salt. 



Calorimeter cup, Exp. 15. 

Any vessel with a polished surface. 


Thermometer, Exp. 3; tumbler, 
Exp. 5; ice or snow. 


13-11 

Cotton gauze. 

Cheesecloth or other piece of cloth. 


Calorimeter cup, Exp. 15. 

Thermometer, Exp. 3. 

Tumbler or beaker. 

14 

Metal tube, pointer and mirror 
scale with support. 

Any standard linear expansion ap¬ 
paratus may be used. 


2 wooden blocks, 20x9x9cm. 

2 ft. of rubber tubing. 3/16 in. 
Steam generator, Exp. 12. 

A flask, or an ether or syrup can, 
with one hole stopper and de -1 
livery tube. 


Micrometer caliper or Vernier cali¬ 
per, Exp. 1. 



Thermometer, Exp. 3; Bunsen burn¬ 
er, Exp. 12; meter stick. 


15 

Calorimeter, complete. 



Steel shot, copper shot, lead shot, 
glass beads, aluminum pellets. 

No one student is expected to use 
all of these substances. 


Balance and weights, Exp. 2; ther¬ 
mometer, Exp. 3; steam generatoi 
and Bunsen burner, Exp. 12. 



16 


Cardboard tube, length about 120 
cm. 

Cork for above tube, bored for ther¬ 
mometer. 

Plug. 

Lead shot, 2 kg. 

Balance and weight, Exp. 1; ther¬ 
mometer, Exp. 3 ; cord. 


Lead pencil. 











































Apparatus 


127 


Exp. No. 


Apparatus 


Suggestions 


17 


18 


19-1 


20 


21 


Acetamide. 

Watch with second hand. 

Beaker, glass, 500 cc. 

Test tube, 5x5/8 in. 

Wire gauze, iron, 6xG in. 

Burette clamp. 

Tripod base, rod and right angle 
clamp, Exp. 3. 

Iron ring, 5 in.; for above. 

Thermometer, Exp. 3; Bunsen burn¬ 
er : co-ordinate paper. 


Boiler of steam generator, Exp. 12. 


Balance and weights, Exp. 1; ther¬ 
mometer, Exp. 3; calorimeter, 
Exp. 15; ring stand and wire 
gauze, Exp. 17; Bunsen burner. 


Pistol and blank cartridges. 
A measuring rod or cord. 


A toy cannon or a sheet of tin and 
an iron rod. 

The distance c-a'ii be gotten from a 
large scale map of the region, it 
one is available. 


j Thermometer, Exp. 3; stop watch, 
I Exp. 9. 

19-11 i Hammer or iron bar. 


2 tuning forks, say 250 and 512. 

Barge flat cork or a strip of leather 
or rubber. 

Glass tul*e, 110x4 cm, Exp. 3. 

Piston for above tube with 110 cm 
handle. 

Clamp, tripod base and rod, Exp. 3. ' 

Thermometer and rubber bands, 
Exp. 3; meter stick. 


wooden support blocks for the 
tube. 


2 bar magnets. 

Piece of soft iron or an iron washer. 


The magnets furnished with the St. 
Louis motor. Exp. 31 may he used. 

In storing magnets, always place 
them so that the unlike poles 
touch each other. 






































128 Laboratory Manual of Physics 


Exp. No. 

Apparatus 

Suggestions 

22-1 

Small bits of glass, iron, steel, gran- 

The materials of Exp. 15 may also 


ulated tin, common “tin,” nickel, 
copper, lead, zinc, wood, paper, 
sand, etc. 

be used. 


Sheets about 8 cm square or larger, 

The sheets of metal of Exp. 11-11 


of cardboard, sheet iron, common 
tin, glass, copper, lead, brass, thin 
wood, etc. 

may also be used. 


Iron filings. 

Bar magnet Exp. 21; Bunsen burn¬ 
er ; copper wire. 

Small iron tacks, Exp. 26. 

22-11 

Half of a hardened steel knitting 

12 cm piece of watchspring. 


needle. 

Note. If a knitting needle is used, 


Compass, graduated, 5 cm diameter. 

render it brittle by heating it to a 
bright red and then dropping it into 


Thread; copper wire; Bunsen 

water. 


burner. 


23-1 

Pith balls. 

Silk thread. 

Glass friction rod. 

Bits of cork. 


Ebonite friction rod. 

Silk pad. 

Flannel pad. 

Sealing wax may be used in place 
of the ebonite rod. 

23-11 

Stirrup replaces the pith balls and 

Make a stirrup out of heavy wire. 


thread of Part I. 

234II 

Leaf electroscope, flask form. 

• 


Proof plane. 

A cent fastened to a stirring rod 
or stick with sealing wax. 

234V 

Metal balls, Exp. 8; electroscope, 
ebonite rod and flannel pad of 
Part I. 


24-1 

Demonstration cell with zinc and 

A complete set of elements for this 


copper elements. 

Galvanoscope frame. 

Sulfuric acid. 

Connecting wire (No. 18 insulated 
copper wire). 

cell will be needed in Exp. 28. 


German silver wire, No. 36. 



Compass, Exp. 22; mercury. 


















Apparatus 


129 


Exp. No. 

Apparatus 

Suggestions 

24-11 

Porous cup. 

Copper sulfate. 

Demonstration cell, compass and 
galvanoscope frame, Part I. 


25 

Knife switch, double pole, double 
throw. 

Daniel cell. 

Copper wire, No. 18, insulated. 
Compass, Exp. 22. 

Commutator. If a switch is used it 
will be found convenient to have 
it mounted on a heavy wooden 
base. 

Dry cell, Exp. 26. 

26-1 

Soft iron core. 

Soft iron horseshoe core. 

Small iron tacks. 

Dry cell. 

Compass, Exp. 22; connecting wire; 
Exp. 24; knife switch, Exp. 25. 

Spike or bolt. 

Bent spike or rod. 

Brads, small nails or iron filings, 
Exp. 22. 

26-11 

D’Arsonval galvanometer. 

Dry cell, Part I; connecting wire, 
Exp. 24. 


27 

Electric bell or buzzer. 

Some form of cell. 

Push button or knife switch of 
Exp. 25. 

Small compass, Exp. 21; connecting 
wire, Exp. 24. 

Dry cell. Exp. 26, is most conven¬ 
ient. 

May he omitted. 

28-1 

Complete set of elements for dem¬ 
onstration cell. 

Nitric, acid. 

Copper sulfate. 

Zinc sulfate. 

Sulfuric acid. 

Hydrochloric acid. 

Galvanoscope frame and compass, 
Exp. 24. 

Demonstration cell, No. 36 German 
silver wire and connecting wire, 
Exp. 24. 

Voltmeter, Exp. Add. 6. is more de¬ 
sirable. 



































130 


Laboratory Manual of Physics 


Exp. No. 

Apparatus 

Suggestions 

28-11 

Galvanoseope frame and compass, 
Exp. 24. 

Demonstration cell, zinc, copper 
elements, sulfuric acid, No. 36 
German silver wire and connect¬ 
ing wire, Exp. 24. 

Voltmeter, Exp. Add. 6. 

An automobile storage battery 
should be examined by the class. 

28-111 

2 dry cells. 



Galvanoseope frame and compass, 
Exp. 24. 

Voltmeter, Exp. Add. 6. 


Connecting wire, Exp. 24. 


29-1 

Wire nails. 



Tumbler, Exp. 5. 



2 dry cells, Exp. 28; sulfuric acid 
and connecting wire, Exp. 24. 

2 or 3 cells are usually needed to 
cause gas to appear at carbon 
plate. 

29-11 

Nickel (coin). 



Tumbler, Exp. 5; copper sulfate and 
connecting wire, Exp. 24; carbon 
rod from demonstration cell and 
2 dry cells, Exp. 28. 


30 

2 coils for induction. 

. 

Home-made coils are very satis¬ 
factory. Use about 40 turns of 
No. 30 insulated copper wire. Bind 
the wire with friction tape, leav- 



ing a 3 cm opening. 


D’Arsonval galvanometer, Exp. 26. 

A galvanoseope may he used, al 
though it is less sensitive. 


Soft iron core, Exp. 26. 

One or more large spikes. 


Dry cell, Exp. 28. 

Bar magnet. Exp. 21; connecting 

Two if available. 


wire. 


31 

St. Louis motor. 

Electromagnet attachment. 

German silver wire, No. 36, Exp. 24. 

A small rheostat is more convenient. 


D’Arsonval galvanometer, Exp. 26. 

Galvanoseope, Exp. 24. 


Dry cell, Exp. 28. 

Thread; connecting wire. 

Two may be needed. 









































Apparatus 


131 


Exp. No. 

Apparatus 

Suggestions 

32 

Plane mirror, blackened. 

Blacken one side of a piece of glass 
with soot from burning camphor 
or a candle. 


Pins. 



Dividers, Exp. 7. 

Protractor. 


Wooden block, Exp. 1 or Exp. 14. 
Rubber bands, Exp. 3; ruler, Exp. 7. 

Note. Each student should work 
alone on this experiment. 

33 

Plate glass prism, euqilateral, with 
sharp edges and corners. 

Dividers and ruler, Exp. 7; pins. 

Note. Each student should work 
alone. 

34 

Screen, unglazed white cardboard 
or paper, with support. 



Dividing screen, cardboard or beav- 
erboard. 



5 similar candles. 



Tripod base and rod, Exp. 3; meter 
stick. 


35-1 

Reading glass of about 15 cm focus. 

Tripod base, rod and right angle 
clamp. Exp. 3; meter stick. 

Note. If a good optical bench is 
available, Part II may be performed 
as a group experiment, with the teach¬ 
er demonstrating. 

35-11 

Screen with a 1 cm circular opening 
covered with wire netting or 
cross wires, mounted on base. 

If the laboratory already possesses 
a screen of wire netting mounted 
on a base, cover the screen with 
a sheet of paper containing a 1 
cm circular opening. 


Source of light consisting of key¬ 
less porcelain receptacle, an 8 
c. p. frosted bulb and an exten¬ 
sion cord with plug. 

Any other source of light. 


White cardboard screen, Exp. 34; 
reading glass and other apparatus 
of Part I. 

J 

A 

36 

Linen tester. 

Substitute for the linen tester a 
pocket magnifier and a black 
screen containing a 1 cm opening. 


Tripod base and rod, Exp. 3; bur¬ 
ette clamp, Exp. 17; meter stick. 


37 

Tripod base, rod and right angle 
clamp, Exp. 3; wooden block, Exp. 
14: white screen, Exp. 34; read¬ 
ing glass, Exp. 35; linen tester 
Exp. 36; meter stick. 



















































132 


Laboratory Manual of Physics 


Exp. No. 

Apparatus 

Suggestions 

Add. 1 

Open-tube manometer, 100 and 25 
cm arms, unmounted. 

Glass tube. 110 cm, with rubber stop¬ 
per and support. Exp. 3; mercury, 
Exp. 4; rubber bands; meter 
stick; co-ordinate paper; gaso¬ 
line. 


Add. 2-1 

Add. 2-11 

Open-tube manometer, 25 and 50 
cm arms. 

Support for above manometer. 

Glass mouthpiece. 

2 in. length of 3/16 in. rubber tub¬ 
ing. 

Barometer and mercury, Exp. 4; 
meter stick. 

U-tube, 25 cm arms. 

Support for U-tube. 

30 cm length of 5 mm glass tubing. 

Glass tube with stopper and sup¬ 
port, Exp. 3; barometer, Exp. 4; 
rubber tubing, Exp. 14; meter 
stick. 

Manometer tube, Exp. Add. 1. 

Tripod base, rod and cord. 

An S cm piece of glass tubing with 
one end rounded in flame. 

Omit barometer, if necessary. 

Manometer, Exp. Add. 1 or Add. 
2-1. 

Grooved block or tripod base, rod 
and clamp. 

Omit barometer, if necessary. 

Note. If practicable, allow the stu¬ 
dent to choose between Part I and 
Part II. 

>- 

CO 

Smooth board, about 120x12 cm. 

Cone-bearing pulley for inclined 
plane. 

Balance and weights, Exp. 1; weight 
hanger, Exp. 6; cord. 

Board, Exp. 11. 

Note. A spring balance may be 
used in place of the weights, hanger 
and pulley, to measure F. A cor¬ 
rection is made for the position of 
the spring balance and, in Part II, 
the effect of friction is eliminated by 
taking the average of the two balance 
readings obtained, first when the car 
is pulled up, and then when it is 
allowed to roll down. 

Add. 4 

Mercury gage for steam generator. 
Pinch cock, screw compressor. 

Steam generator. Exp. 12. 
Thermometer, Exp. 3; mercury; 
Bunsen burner ; meter stick. 

Glass tubing bent in U form. 

Flask with a three hole stopper. 

Add. 5 

Sonometer, complete. 

2 or more tuning forks and a large 
flat cork, Exp. 20. 































Apparatus 


133 


Exp. No. 

Apparatus 

Suggestions 

Add. 6 

Lengths of Nos. 24 and 30 German 
silver wire, No. 30 iron wire and 
No. 30 copper wire, mounted on 
a board. 

D.C. voltmeter, 5 volt. 

D.C. ammeter, 3 amperes. 

Knife switch, Exp. 25; 2 dry cells, 
Exp. 28. 

Loose wire may be used but it is 
more convenient to have the wires 
strung between 4 pairs of binding 
posts mounted on a 1-m board as 
in Fig. 50. 

Note. Because of the cost of the 
apparatus, this experiment may be 
performed by the class as a group, 
with the instructor demonstrating. 
Note that Exp. Add. 7 practically du¬ 
plicates B and C of this experiment. 

Add. 7 

Wheatstone bridge. 

Resistance box, plug type. 

Resistance wires, Exp. Add. 6. 

Knife switch, Exp. 25; D’Arsonval 
galvanometer, Exp. 20; dry cell, 
Exp. 28; connecting wire. 

See footnote to experiment. 

These wires may be loose, mounted 
on a board, or wound on blocks 
or spools. 

Add. S 

• 

1 2 tungsten lamps, 25-watt and 40- 
watt. 

2 carbon filament lamps 16 c. p. and 
32 c. p. 

Keyless porcelain receptacle and ex¬ 
tension cord, Exp. 35. 

A. C. Voltmeter, 150-75 volts. 

A. C. Ammeter, 7.5 amperes. 

60-watt, 100-watt or 200-watt lamps. 

Add. 9 

Electric irons, toasters, grills, curl¬ 
ing irons, fans, sewing machine 
motors, etc., to be borrowed lo¬ 
cally. 

Immersion coil. 

Thermometer, Exp. 3 ; knife switch, 
Exp. 25; A. C. ammeter and A. G. 
voltmeter, Exp. Add. 8; connect¬ 
ing wire. 

Students should be encouraged to 
bring electrical appliances from 
their homes. 

May be omitted. 

Add. 10 

Bunsen photometer screen and sup¬ 
port or a Bunsen photometer box. 

Gandle holder, single. 

Candle holder, quadruple. 

Support blocks for meter stick. 

5 similar candles, Exp. 34; meter 
stick. 

See footnote to this experiment. 

* 























































< 





























































































I 




•» 











4 











































































